Foraminifera 



DRY WIIOLEMOUNTS 



17 



Specific Example 



Preparation of a Strewn Slide of Foraminifera or Radiolaria 



Wholemoiints of the dried tests of 

 Foraminifera, either fossil or I'cccut, are 

 customarily referred to as strewn slides 

 even though the individual tests may be 

 arranged in place. Tests of Foraminifera 

 may be obtained either from sand, from 

 marine sludge, or from fossil deposits, and 

 the method by which the shells are sepa- 

 rated is in each instance different. Radio- 

 laria are almost invariably obtained from 

 fossil deposits. 



Forminiferal sands, which may be pur- 

 chased or collected, are the best source of 

 material. Large numbers of shells are 

 thrown onto beaches in many parts of the 

 world where they form whitish ridges. If 

 such a ridge is observed it is only necessary 

 to scoop off the surface with a spoon and 

 to preserve it for further examination. 

 Many scientific supply houses sell these 

 sands. The separation of the dried shells 

 from the sand is relatively simple. The 

 whole may either be sprinkled onto the 

 surface of a large vessel of cold water, in 

 which case the majority of the shells will 

 float, since they are filled with air; or 

 carbon tetrachloride, the high specific 

 gravity of which ensures that Foraminif- 

 eral shells with only a small quantity of 

 air enclosed will rise to the surface, may 

 be substituted for water. If only a few 

 shells are required they may be picked 

 from the surface of the flotation medium 

 with a brush and laid to dry on a disk of 

 fine filter paper. If all the shells are re- 

 quired, the surface layer containing the 

 floating shells should be poured off through 

 a fine sieve. Bolting silk is the best mate- 

 rial from which to prepare this sieve, 

 though fine brass screen wire may also be 

 used. 



The separation of tests from marine 

 deposits dredged from the bottom is not 

 so easy since they are, in this case, mixed 

 not only with particles of sand but also 

 with considerable quantities of fine sludge 

 which may include some organic matter. 

 If the sludge is free from organic matter, 

 the mass may be passed through a series 



of sieves under a jet of running water; but 

 even under those circumst:iiices the tests 

 will usually be discolored. Laporte 1946, 

 p. 194 recommends that such tests be 

 boiled in an alkaline solution of calcium 

 hypochlorite {eau de Javelle) which serves 

 the double purpose of bleaching the stains 

 and removing any trace of organic matter 

 which may remain. If the sludge is heavily 

 contaminated with organic matter, the 

 mass should be boiled for some time in a 

 weak (2%) solution of sodium or potas- 

 sium hydroxide before being sieved. Cush- 

 man 1940, p. 26 suggests also that tests 

 may be separated as the old gold miners 

 separated gold by rotating the mass in a 

 flattened dish. The Foraminifera, being 

 somewhat lighter than the rest of the 

 material present, will collect round the 

 edges of the dish, from which they may be 

 jerked with a circular motion. 



Tests of fossil Foraminifera may^be ob- 

 tained from sandy deposits. They are also 

 found embedded in clay deposits, or chalk. 

 Foraminifera concreted in limestone can- 

 not, in most cases, be removed and made 

 into wholemounts. Foraminiferal tests ob- 

 tained from sandy deposits may be sepa- 

 rated by flotation in the manner already 

 described, but these shells are usually 

 dirty either from chalk or clay, and must 

 be thoroughly cleaned if a satisfactory 

 mount is to be prepared. It is best to 

 boil them after they have been separated 

 in a 5% solution of sodium carbonate. 

 After they have boiled for some time the 

 beaker containing them is removed from 

 the flame and the Foraminifera are al- 

 lowed to settle to the bottom. The cloudy 

 alkahne solution is then poured off and re- 

 placed with fresh solution and this is re- 

 peated until the tests are sufficiently clean. 

 If the dirt is particularly tenacious, it 

 may often be loosened by boiling the tests 

 in a relatively small quantity of the 

 alkaline solution which is then poured 

 while boiling over a mass of cracked ice. 

 This sudden temperature change will often 

 loosen dirt which cannot be removed by 



