CLASSIFICATION OP ANIMALS. 



exist, so that no analogous example can be found among the 

 living classes. In such cases we shall endeavour, as often as 

 convenient, to accompany the technical name with a figured 



Fig. 138. Fig. 139. Fig. 140. 



Examples of Amorphozoa. 



Fig. 138. Clioua Duvernoyi (fossil). Fig. ] 3d. Cribrospongia reticulata 



Fig. 140. A part magnified. 



representation of some species of the object. In the preceding 

 table we have accordingly given some familiar examples, and we 

 now annex figured illustrations of those objects which may be 

 supposed to be the least familiar to ordinary readers. The 

 references to the figures are given in the table.* 



248. Geologists have ascertained the existence of the organic 

 remains of about twenty-four thousand species of the different 

 orders of animals, which they have assigned to upwards of 

 fourteen hundred and seventy genera. Of these numerous 

 species none survive ; but we find in the existing animal king- 

 dom about five hundred and fifty of the genera, the remainder 

 being extinct. 



249. Until very recently, it was considered by geologists that 

 those twenty-four thousand fossil species were distributed through 

 the strata of the crust of the earth, in such a manner that the 

 great majority of them should be common to strata of very 

 different dates of deposition, and that comparatively few were 

 exclusively found in strata of a particular date ; these few being 

 consequently called characteristic species, inasmuch as they 

 supplied to geologists certain tests, by which the dates of strata 

 left uncertain from their mineral ogical character could be fixed. 

 The elaborate researches of M. D'Orbigny, who has catalogued, 



* We must warn the reader that he must not understand that the 

 example given in the parenthesis is in every case an individual of the 

 species, or even of the genus, of the object to be illustrated. It will be 

 more generally a specimen of the class or order. This is the only expe- 

 dient that I can devise to popularise this part of our subject. 



87 



