J. Forens. Sci. Soc. (1977), 17, 81
Diatoms in Forensic Science A. J. PEABODY Home Ofie Forensic Science Laboratory, Washington Hall, Euxton, Chorley, Lancashire, England PR7 6HJ The occurrence, methods of treatment, and applications of diatoms inforensic science are described. Reference is made to drowning incidmts, and other cases whefe the we of diatom by the forensic scientist can provide valuable evidence.
Introduction Diatoms are microscopic unicellular algae of the Class Bacillariophyceae, which is usually included in the Chrysophyta. They range in size from less than 5p to more than 5 0 0 ~There . is great variety in the shape of the cell, and in the ornamentation of the silica cell walls (Figure 1). Exceptionally, some tropical species are visible to the naked eye. Diatoms have a world-wide distribution, and are to be found almost anywhere where there is light, moisture, and nutrients. They can use extreme habitats, e.g., streams running off slag heaps with a pH lower than 2, and thermal springs with average temperatures higher than 50°C. Their most usual habitats however, are seas, lakes, rivers and ditches, and also damp walls, soil surfaces and other damp places. In addition to these living forms there are large deposits of fbssil diatoms, formed by the slow accumulation of dead cells on the floors of lakes and seas. Some of the largest of these deposits of diatomaceous earth, or kieselguhr, are in the Lompoc area of California, where the beds are several hundred feet thick over wide areas, and which date from about 25 million years ago. Other important sources are Algeria, Kenya and Germany. In the British Isles, there is still a small deposit being worked at Kentmere, and another in Northern Ireland. Other deposits in the Hebrides and Aberdeenshire have been abandoned. Because of the small size of the individual cells and the compressed nature of the material, one cubic inch of diatomite may contain between 25-50 million cells. Because of its excellent absorbing qualities, diatomaceous earth was originally used in the manufacture of dynamite, acting as an inert carrier for nitroglycerine. Its use today is mainly in water filtration plants, as a filter in the manufacture of asbestos sheets, and as an extender in paints. It is used as an inert filler in safety-match heads, and its mildly abrasive properties account for its use in car polishes. It has apparently been used as a base for cosmetic face powder, but examination of 15 face powders for diatoms proved negative. It has been used as an adulterant in flour during wartime and wheat shortage and thereby incorporated into bread. Identification There are upwards of 10,000 different diatom species, both living and fossil, and in addition, many of these have sub-species. Hendey (1954) lists 771 species in 104 genera for British coastal waters alone. As one may suppose, this creates difficulties in the identification of species, which is very complex. These difficulties are compounded by the standard reference works being in German and often difficult to acquire (Hustedt, 1930a; 1930b). The latter work is incomplete but remains the standard text. Diatoms of British coastal waters are well covered by Hendey (1964), and a beginning has been made on the North
American Flora (Patrick and Reimer, 1966). These two works are in English. From a practical point of view, the identification of diatom species is difficult without specialised knowledge. For this reason, the determination of diatom species should be attempted only by experienced diatomologists. Preparation Because of the inert nature of the silica cell walls of diatoms, rigorous chemical treatment (for routine microscopy) is possible without affecting the apparently delicate structure. Less drastic treatment is required however, for examination by SEM. The object of any such preparation is the removal of all organic and inorganic material except silica, thereby leaving the diatom cells unobscured by cell contents and general debris, which would otherwise hinder identification. The basic cleaning procedure is as follows :1. Remove CaCO, and other carbonates with conc. HC1. (The subsequent addition of conc. H,SO, would otherwise result in the production of sparingly soluble CaSO,) Wash. Discard supernatant. 2. Add conc. H,SO, to residue. Boil until suspension turns black. (Conc. H,SO, chars organic material present). 3. Allow to cool to about 50°C. Add solid NaNO, and reheat until suspension turns brown and finally clears. (HNO, produced oxidises the carbon to CO,). 4. The resultant suspension of silica diatom cells must be thoroughly washed in diatom free water and resuspended in acetone. Sand particles may be present in some samples, and the larger grains can be removed by allowing them to settle for a few seconds and rapidly decanting. This procedure is conveniently carried out on a semi-micro scale in new 3" x #" tubes, but where the material being examined is bulky (e.g., bone marrow) or explosive (e.g., match heads), other suitable vessels are necessary for the initial stages. Mounting Because cleaned diatom cells are colourless and transparent, and have a refractive index of 1-44, mounting in water (R.I. = 1-33), glycerine jelly (R.I. = 1-41)or Canada balsam (R.I. = 1-51) will not reveal the fine structure of the cells necessary for identification. Several synthetic resins are commercially available, e.g., Styrax (R.I. = 1.63) and Clearax (R.I. = 1-66), but the most suitable is Naphrax (R.I. = 1.74). Mountants with a higher refractive index give superlative resolution, but are usually difficult to prepare, made from toxic compounds, or only temporary mounts, e.g., Realgar (R.I. = 2-00). In any case, the resolution attained using Naphrax is sufficient for most purposes. Naphrax is available from N.B.S., 3 1 Cheltenham Ave., Ipswich. The diatom suspension in acetone is spotted onto a size 0 coverslip and dried. A small drop of Naphrax on a slide is heated slightly to melt the mountant. The coverslip is placed on the mountant and heated over a bunsen burner until bubbles cease, or until the edges begin to smoke. Bubbles trapped in the mount will rapidly disappear, and the mount is rock hard, permanent, and ready for use almost immediately. The mountant can be remelted if required, for instance if the slide is broken, and the coverslip remounted. Since in many cases the number of diatoms on one slide may be very low, it is an advantage to make the area of dried suspension as small as possible. In control samples, where the-number of diatoms is likely to be larger, there need be no such-restriction in area.
Diatoms in Casework Diatoms and Drowning The diagnosis of death by drowning is not usually difficult in cases where 83
the body is recovered soon after death. In cases where the pathologist would like confirmation of his diagnosis, or when the body is decomposed or dismembered, samples may be submitted for diatom examination. The mechanics of drowning are dealt with by Timperman (1972). Suffice it to say that as a person drowns, the lungs aspirate more and more water, until finally the lungs are filled with water. This creates great pressure, and the peripheral alveoli of the lungs rupture, admitting water (and its contents) to the blood system. This water is circulated round the body until death. Diatoms present in this water are also carried round, and therefore reach many parts of the body. The presence of diatoms in enclosed tissue, e.g., bone marrow is an indication that drowning has occurred. Lungs and stomach contents are unsuitable because diatoms may occur in the lungs through means other than drowning, and stomach contents may contain diatoms from sources other than water swallowed during drowning. I t had been the practice previously to ask for the sternum to be submitted, since this is removed as a routine during post-mortem examination. I t became apparent however, that the high ratio of bone to bone marrow resulted in large amounts of insoluble calcium salts remaining after the cleaning process, since the marrow could not be separated from the bone before starting the treatment. A more satisfactory method is to use a long bone, usually the femur. I t is a relatively easy task to remove the marrow from a 19" long cylinder of bone. The advantage is that there is no bone to interfere with the cleaning process. Bone marrow has a high fat content, and it is useful, therefore to defat first with petroleum ether. This will usually decrease the volume by about 114, and will also avoid complications later in the cleaning process due to a floating layer of fat. Defatting is particularly important where adipocere has been formed. The number of diatoms found in bone marrow in cases of drowning is always very small, but the presence of diatoms in bone marrow is indicative of death by drowning. The absence of diatoms does not mean however, that drowning has not occurred, since drowning may have happened in water containing very few or no diatoms, e.g., tap water. In these circumstances, diatoms may not enter the body in sufficient numbers to be found in the bone marrow sample. Because of the low numbers which are used as an indication of drowning, it is imperative that a blank control of reagents be run at the same time, and that precautions be taken at the post-mortem examination against any possible contamination of the enclosed tissues by water on the surface of the body. The type of diatoms present in the bone marrow will indicate whether the deceased drowned in fresh or salt water. If diatoms are present in sufficiently high numbers (say 10-15), it may be possible to give an opinion as to where drowning occurred. Case Example. An elderly man disappeared from home at Christmas. 49 years later the lower part of a skeleton was found in a nearby stream, the feet still shod in the missing man's distinctive shoes. The small amount of flesh remaining on the bones, and the marrow of the femur were converted to adipocere, and the inside of the bone appeared quite dry. An examination of the bone marrow revealed several diatoms of a type likely to be found in an upland stream, and indicated death by drowning. Case Example. The stomach contents of a recently drowned woman were submitted for diatom examination. The body was washed up on a beach, and it was thought that diatoms in the stomach would throw light on where the woman had drowned. There were indeed many diatoms from an estuarine habitat present, and this would have supported the suggestion that she drowned in the Mersey. Further investigation of these stomach contents revealed however, the remains of shellfish, e.g., cockles and winkles, which are surface grazers of seaweeds. Seaweeds often have a coating of diatoms upon them, and diatoms
may therefore be found in the gut of these shellfish. It was not possible therefore, in this instance, to draw any conclusions in connection with the drowning.
Diatoms in Safe Ballasts Many safe manufacturers used diatomaceous earth as a safe ballast during the early part of this century. Many of these safes are still in use, sometimes renovated so that they appear fairly new. One firm was using this material as a safe ballast as recently as 1966. Diatomaceous earth is an excellent insulator but offers no extra security in safes, and its use is now discontinued. From a forensic point of view, diatomaceous safe ballast in cases of safe breaking can provide valuable evidence (Peabody, 1971). About half of the diatomaceous safe ballasts examined at this laboratory appear to have a common origin, and resemble a diatomaceous earth deposit in N. Germany. I t would be unlikely for a person to have this material on his clothing through innocent casual contact. The dry powdery nature of this type of safe ballast is such that any person forcibly opening a safe containing this material would certainly transfer some onto his clothing. Only a very small smear on a garment is needed to characterise it as originating from a particular safe ballast, but the diatomaceous earth has often been ground, and the cells are therefore fragmented (Figure 2). Case Example. Three men were alleged to have broken into a safe containing diatomaceous earth safe ballast. Two of the men confessed to the offence and incriminated the third who strenuously denied it. These men spoke of c c ~ l ~ ~ of white powder" coming out of the back of the safe. On the left shoulder of the third man was a thin white smear of diatomaceous earth, containing several hundred cells. This diatomaceous earth was similar in all respects to the safe ballast and all three men were convicted. Diatoms in Polishes Car polish often contains diatomaceous earth, metal polish less frequently. Car polishes made by the same manufacturer may contain completely different diatoms, and this offers a means of differentiation between them. Case Example. A sports car was parked in a cinema car-park next to a similar car. When the owner returned, his hub caps were missing, and the second car had left. The owner of this car was traced and the hub caps from his vehicle submitted for examination. These bore traces of car polish containing diatoms of types found in the aggrieved's car polish, but not in the suspect's car polish, which was a completely different type. Care Example. &1,000 worth of silver was stolen from a house, and the only item recovered, (from a suspected receiver), was a silver candelabrum, the same design as one left behind by the thieves. The owner was unable to confirm that they constituted a pair. Both candelabra had had the same polishing treatment by the owner, who used a metal polish containing diatomaceous earth. Traces of this were found on both candelabra, indicating a common origin, and that the receiver was indeed holding stolen goods. Diatoms in Other Materials Matches. Out of 12 different brands of safety matches, 9 contained diatomaceous earth, 3 did not. I t is possible to distinguish different brands on the basis of the type of diatoms contained in the head. Enough diatoms are present in a single match head for comparison purposes, they are undamaged by heat, and can still be recovered after the match has been struck. Paints. Diatomaceous earth is used as an extender and flatting agent in emulsion paints, undercoats, flat lacquers and enamels, and masonry and traffic paints. The presence of diatoms may be useful when comparing small flakes of paint. I n order to see diatoms in dried paint, however, it must be dissolved in suitable solvents. There are no non-destructive methods of examining diatoms in paint.
Figure 2.
Diatomaceous safe ballast showing its fragmented nature.
Soils. Diatoms do occur quite commonly on soil surfaces (Lund, 1945) but are of only limited use in soil comparisons, since essentially the same species are met with on nearly all soils. There are certainly differences in numbers of the various species making up the soil community, but these differences are insufficient basis for comparison by the forensic scientist. References HENDEY, N. I., 1954,J. mar. biol. Ass. U.K., 33, 537-560. N. I., 1964, Fishery Investigations Series IV. An Introductory account HENDEY, of the smaller algae of British Coastal Waters Part V: Bacillariophyceae,
H.M.S.O. HUSTEDT, F., 1930a, Die Siisswasser-Flora Mitteleuropas (ed. Pascher), 10, Bacillariophyta, Jena.
HUSTEDT, F., 1930b, Die Kieselalgen Deutschlands, 0sterreich.s und der Schweiz mit Beruchsichtigung der ubrigen Lhder Europas sowie der angrwenden Meeresgebiete. In Rabenhorst's Kryptogamen-Flora, 7, Leipzig. J. W. G., 1945, New Phytologist, 44, 196-219. LUND, PATRICK, R. and REIMER,C. W., 1966, Diatoms of the United States Vol. I. Acad. Nut. Sci. Phil., 13. PEABODY, A. J., 1971, J. Forens. Sci. Soc., 11, 227. TIMPERMAN, J., 1972, Forens. Sci., 1, 397-409.
Diatoms in Forensic Science A. J. PEABODY Home Ofie Forensic Science Laboratory, Washington Hall, Euxton, Chorley, Lancashire, England PR7 6HJ The occurrence, methods of treatment, and applications of diatoms inforensic science are described. Reference is made to drowning incidmts, and other cases whefe the we of diatom by the forensic scientist can provide valuable evidence.
Introduction Diatoms are microscopic unicellular algae of the Class Bacillariophyceae, which is usually included in the Chrysophyta. They range in size from less than 5p to more than 5 0 0 ~There . is great variety in the shape of the cell, and in the ornamentation of the silica cell walls (Figure 1). Exceptionally, some tropical species are visible to the naked eye. Diatoms have a world-wide distribution, and are to be found almost anywhere where there is light, moisture, and nutrients. They can use extreme habitats, e.g., streams running off slag heaps with a pH lower than 2, and thermal springs with average temperatures higher than 50°C. Their most usual habitats however, are seas, lakes, rivers and ditches, and also damp walls, soil surfaces and other damp places. In addition to these living forms there are large deposits of fbssil diatoms, formed by the slow accumulation of dead cells on the floors of lakes and seas. Some of the largest of these deposits of diatomaceous earth, or kieselguhr, are in the Lompoc area of California, where the beds are several hundred feet thick over wide areas, and which date from about 25 million years ago. Other important sources are Algeria, Kenya and Germany. In the British Isles, there is still a small deposit being worked at Kentmere, and another in Northern Ireland. Other deposits in the Hebrides and Aberdeenshire have been abandoned. Because of the small size of the individual cells and the compressed nature of the material, one cubic inch of diatomite may contain between 25-50 million cells. Because of its excellent absorbing qualities, diatomaceous earth was originally used in the manufacture of dynamite, acting as an inert carrier for nitroglycerine. Its use today is mainly in water filtration plants, as a filter in the manufacture of asbestos sheets, and as an extender in paints. It is used as an inert filler in safety-match heads, and its mildly abrasive properties account for its use in car polishes. It has apparently been used as a base for cosmetic face powder, but examination of 15 face powders for diatoms proved negative. It has been used as an adulterant in flour during wartime and wheat shortage and thereby incorporated into bread. Identification There are upwards of 10,000 different diatom species, both living and fossil, and in addition, many of these have sub-species. Hendey (1954) lists 771 species in 104 genera for British coastal waters alone. As one may suppose, this creates difficulties in the identification of species, which is very complex. These difficulties are compounded by the standard reference works being in German and often difficult to acquire (Hustedt, 1930a; 1930b). The latter work is incomplete but remains the standard text. Diatoms of British coastal waters are well covered by Hendey (1964), and a beginning has been made on the North
American Flora (Patrick and Reimer, 1966). These two works are in English. From a practical point of view, the identification of diatom species is difficult without specialised knowledge. For this reason, the determination of diatom species should be attempted only by experienced diatomologists. Preparation Because of the inert nature of the silica cell walls of diatoms, rigorous chemical treatment (for routine microscopy) is possible without affecting the apparently delicate structure. Less drastic treatment is required however, for examination by SEM. The object of any such preparation is the removal of all organic and inorganic material except silica, thereby leaving the diatom cells unobscured by cell contents and general debris, which would otherwise hinder identification. The basic cleaning procedure is as follows :1. Remove CaCO, and other carbonates with conc. HC1. (The subsequent addition of conc. H,SO, would otherwise result in the production of sparingly soluble CaSO,) Wash. Discard supernatant. 2. Add conc. H,SO, to residue. Boil until suspension turns black. (Conc. H,SO, chars organic material present). 3. Allow to cool to about 50°C. Add solid NaNO, and reheat until suspension turns brown and finally clears. (HNO, produced oxidises the carbon to CO,). 4. The resultant suspension of silica diatom cells must be thoroughly washed in diatom free water and resuspended in acetone. Sand particles may be present in some samples, and the larger grains can be removed by allowing them to settle for a few seconds and rapidly decanting. This procedure is conveniently carried out on a semi-micro scale in new 3" x #" tubes, but where the material being examined is bulky (e.g., bone marrow) or explosive (e.g., match heads), other suitable vessels are necessary for the initial stages. Mounting Because cleaned diatom cells are colourless and transparent, and have a refractive index of 1-44, mounting in water (R.I. = 1-33), glycerine jelly (R.I. = 1-41)or Canada balsam (R.I. = 1-51) will not reveal the fine structure of the cells necessary for identification. Several synthetic resins are commercially available, e.g., Styrax (R.I. = 1.63) and Clearax (R.I. = 1-66), but the most suitable is Naphrax (R.I. = 1.74). Mountants with a higher refractive index give superlative resolution, but are usually difficult to prepare, made from toxic compounds, or only temporary mounts, e.g., Realgar (R.I. = 2-00). In any case, the resolution attained using Naphrax is sufficient for most purposes. Naphrax is available from N.B.S., 3 1 Cheltenham Ave., Ipswich. The diatom suspension in acetone is spotted onto a size 0 coverslip and dried. A small drop of Naphrax on a slide is heated slightly to melt the mountant. The coverslip is placed on the mountant and heated over a bunsen burner until bubbles cease, or until the edges begin to smoke. Bubbles trapped in the mount will rapidly disappear, and the mount is rock hard, permanent, and ready for use almost immediately. The mountant can be remelted if required, for instance if the slide is broken, and the coverslip remounted. Since in many cases the number of diatoms on one slide may be very low, it is an advantage to make the area of dried suspension as small as possible. In control samples, where the-number of diatoms is likely to be larger, there need be no such-restriction in area.
Diatoms in Casework Diatoms and Drowning The diagnosis of death by drowning is not usually difficult in cases where 83
the body is recovered soon after death. In cases where the pathologist would like confirmation of his diagnosis, or when the body is decomposed or dismembered, samples may be submitted for diatom examination. The mechanics of drowning are dealt with by Timperman (1972). Suffice it to say that as a person drowns, the lungs aspirate more and more water, until finally the lungs are filled with water. This creates great pressure, and the peripheral alveoli of the lungs rupture, admitting water (and its contents) to the blood system. This water is circulated round the body until death. Diatoms present in this water are also carried round, and therefore reach many parts of the body. The presence of diatoms in enclosed tissue, e.g., bone marrow is an indication that drowning has occurred. Lungs and stomach contents are unsuitable because diatoms may occur in the lungs through means other than drowning, and stomach contents may contain diatoms from sources other than water swallowed during drowning. I t had been the practice previously to ask for the sternum to be submitted, since this is removed as a routine during post-mortem examination. I t became apparent however, that the high ratio of bone to bone marrow resulted in large amounts of insoluble calcium salts remaining after the cleaning process, since the marrow could not be separated from the bone before starting the treatment. A more satisfactory method is to use a long bone, usually the femur. I t is a relatively easy task to remove the marrow from a 19" long cylinder of bone. The advantage is that there is no bone to interfere with the cleaning process. Bone marrow has a high fat content, and it is useful, therefore to defat first with petroleum ether. This will usually decrease the volume by about 114, and will also avoid complications later in the cleaning process due to a floating layer of fat. Defatting is particularly important where adipocere has been formed. The number of diatoms found in bone marrow in cases of drowning is always very small, but the presence of diatoms in bone marrow is indicative of death by drowning. The absence of diatoms does not mean however, that drowning has not occurred, since drowning may have happened in water containing very few or no diatoms, e.g., tap water. In these circumstances, diatoms may not enter the body in sufficient numbers to be found in the bone marrow sample. Because of the low numbers which are used as an indication of drowning, it is imperative that a blank control of reagents be run at the same time, and that precautions be taken at the post-mortem examination against any possible contamination of the enclosed tissues by water on the surface of the body. The type of diatoms present in the bone marrow will indicate whether the deceased drowned in fresh or salt water. If diatoms are present in sufficiently high numbers (say 10-15), it may be possible to give an opinion as to where drowning occurred. Case Example. An elderly man disappeared from home at Christmas. 49 years later the lower part of a skeleton was found in a nearby stream, the feet still shod in the missing man's distinctive shoes. The small amount of flesh remaining on the bones, and the marrow of the femur were converted to adipocere, and the inside of the bone appeared quite dry. An examination of the bone marrow revealed several diatoms of a type likely to be found in an upland stream, and indicated death by drowning. Case Example. The stomach contents of a recently drowned woman were submitted for diatom examination. The body was washed up on a beach, and it was thought that diatoms in the stomach would throw light on where the woman had drowned. There were indeed many diatoms from an estuarine habitat present, and this would have supported the suggestion that she drowned in the Mersey. Further investigation of these stomach contents revealed however, the remains of shellfish, e.g., cockles and winkles, which are surface grazers of seaweeds. Seaweeds often have a coating of diatoms upon them, and diatoms
may therefore be found in the gut of these shellfish. It was not possible therefore, in this instance, to draw any conclusions in connection with the drowning.
Diatoms in Safe Ballasts Many safe manufacturers used diatomaceous earth as a safe ballast during the early part of this century. Many of these safes are still in use, sometimes renovated so that they appear fairly new. One firm was using this material as a safe ballast as recently as 1966. Diatomaceous earth is an excellent insulator but offers no extra security in safes, and its use is now discontinued. From a forensic point of view, diatomaceous safe ballast in cases of safe breaking can provide valuable evidence (Peabody, 1971). About half of the diatomaceous safe ballasts examined at this laboratory appear to have a common origin, and resemble a diatomaceous earth deposit in N. Germany. I t would be unlikely for a person to have this material on his clothing through innocent casual contact. The dry powdery nature of this type of safe ballast is such that any person forcibly opening a safe containing this material would certainly transfer some onto his clothing. Only a very small smear on a garment is needed to characterise it as originating from a particular safe ballast, but the diatomaceous earth has often been ground, and the cells are therefore fragmented (Figure 2). Case Example. Three men were alleged to have broken into a safe containing diatomaceous earth safe ballast. Two of the men confessed to the offence and incriminated the third who strenuously denied it. These men spoke of c c ~ l ~ ~ of white powder" coming out of the back of the safe. On the left shoulder of the third man was a thin white smear of diatomaceous earth, containing several hundred cells. This diatomaceous earth was similar in all respects to the safe ballast and all three men were convicted. Diatoms in Polishes Car polish often contains diatomaceous earth, metal polish less frequently. Car polishes made by the same manufacturer may contain completely different diatoms, and this offers a means of differentiation between them. Case Example. A sports car was parked in a cinema car-park next to a similar car. When the owner returned, his hub caps were missing, and the second car had left. The owner of this car was traced and the hub caps from his vehicle submitted for examination. These bore traces of car polish containing diatoms of types found in the aggrieved's car polish, but not in the suspect's car polish, which was a completely different type. Care Example. &1,000 worth of silver was stolen from a house, and the only item recovered, (from a suspected receiver), was a silver candelabrum, the same design as one left behind by the thieves. The owner was unable to confirm that they constituted a pair. Both candelabra had had the same polishing treatment by the owner, who used a metal polish containing diatomaceous earth. Traces of this were found on both candelabra, indicating a common origin, and that the receiver was indeed holding stolen goods. Diatoms in Other Materials Matches. Out of 12 different brands of safety matches, 9 contained diatomaceous earth, 3 did not. I t is possible to distinguish different brands on the basis of the type of diatoms contained in the head. Enough diatoms are present in a single match head for comparison purposes, they are undamaged by heat, and can still be recovered after the match has been struck. Paints. Diatomaceous earth is used as an extender and flatting agent in emulsion paints, undercoats, flat lacquers and enamels, and masonry and traffic paints. The presence of diatoms may be useful when comparing small flakes of paint. I n order to see diatoms in dried paint, however, it must be dissolved in suitable solvents. There are no non-destructive methods of examining diatoms in paint.
Figure 2.
Diatomaceous safe ballast showing its fragmented nature.
Soils. Diatoms do occur quite commonly on soil surfaces (Lund, 1945) but are of only limited use in soil comparisons, since essentially the same species are met with on nearly all soils. There are certainly differences in numbers of the various species making up the soil community, but these differences are insufficient basis for comparison by the forensic scientist. References HENDEY, N. I., 1954,J. mar. biol. Ass. U.K., 33, 537-560. N. I., 1964, Fishery Investigations Series IV. An Introductory account HENDEY, of the smaller algae of British Coastal Waters Part V: Bacillariophyceae,
H.M.S.O. HUSTEDT, F., 1930a, Die Siisswasser-Flora Mitteleuropas (ed. Pascher), 10, Bacillariophyta, Jena.
HUSTEDT, F., 1930b, Die Kieselalgen Deutschlands, 0sterreich.s und der Schweiz mit Beruchsichtigung der ubrigen Lhder Europas sowie der angrwenden Meeresgebiete. In Rabenhorst's Kryptogamen-Flora, 7, Leipzig. J. W. G., 1945, New Phytologist, 44, 196-219. LUND, PATRICK, R. and REIMER,C. W., 1966, Diatoms of the United States Vol. I. Acad. Nut. Sci. Phil., 13. PEABODY, A. J., 1971, J. Forens. Sci. Soc., 11, 227. TIMPERMAN, J., 1972, Forens. Sci., 1, 397-409.
