METHOD OF PREPARING FOSSIL INFUSORIA. 397 



One remarkable fact, in connection with fossil Infusoria, is 

 that most of the forms may be still found in the recent state. 

 The beautiful engine-turned discs (Coscinodisci), so abundant 

 m the Richmond earth, may be met with in our own seas, also 

 in great profusion in the deposits of Guano on the African 

 and American coasts, and even in the stomachs of the oyster, 

 scallop, and other molluscous animals so common on all our 

 shores. 



Method of Preparing Fossil Infusoria. — A great number of 

 the infusorial earths may be mounted up as objects without 

 any previous washing or other preparation, by the method 

 described at page 309, but some, such as chalk, must be 

 repeatedly washed to deprive the infusoria of all impurities ; 

 whilst others, and these by far the most numerous, require 

 either to be digested for a long time, or even boiled in strong 

 nitric or hydrochloric acids for the same purpose. Supposing 

 the earth about to be prepared be some of that from Richmond, 

 in America, a small portion having been placed in a test 

 tube (or other convenient vessel capable of bearing the heat 

 of a lamp), enough diluted hydrochloric acid is to be poured 

 upon it to fill about half the tube, brisk effervescence wiU 

 now take place, which may be assisted by the application of a 

 small amount of heat, either from a sand-bath or from a lamp ; 

 as soon as the action of the acid has ceased, another supply 

 may be added, and the same continued until no further effect 

 is produced ; strong nitric acid should now be substituted for 

 the hydrochloric, when a further effervescence will take place, 

 which also may be greatly aided by heat ; after two or three 

 fresh supplies of this acid, distilled water should be employed to 

 dilute the remains of the acid in the tube, and this repeated 

 untU the water comes away perfectly clear and without any 

 trace of acidity; the residue of the earth, which consists of 

 silica, will contain all the infusorial forms, some of thig 

 may be taken up by a fishing-tube, laid on a slide, and ex- 

 amined in the usual way ; should perfect specimens be present, 

 they may be mounted in Canada balsam in the manner 

 described in page 309 ; if not, the slide may be wiped clean, 

 and another portion of the sediment taken, and dealt with in 



