114 LIFE ON THE EARTH. 



eroded beaks, plications on the surface, reflexions of 

 the lip, carinations of the whorls, which cannot be 

 consistent with accumulated tendencies to change. 



If in the same spirit we look at the few Insects, 

 Crustaceans and Reptilia, the only other freshwater 

 groups which occur at several stages in the series 

 of strata, very similar results appear, much resem- 

 blance in the components of the several groups from 

 whatever age we take the examples, and much ad- 

 herence to generic, or family type. The series of 

 freshwater Reptiles considered in this respect offers 

 the largest variations, but the structure of some of 

 the earlier forms, even in regard to the limbs, is too 

 uncertain to allow of a satisfactory view of their 

 history. True Crocodiles with vertebrae concave in 

 front, occur above the Chalk 1 ; Crocodilians with bi- 

 concave vertebrae an indication of aquatic, but not 

 necessarily of marine life are frequent in the Oolites 

 and Lias 2 , and are found in the Trias 3 , with a La- 

 certian fossil*. Batrachians with deciduous gills are 

 mostly of Tertiary date 5 , but allies of the Menopoma 

 and Triton appear in the Coal Strata, and in Strata 

 above them 6 . Freshwater Chelonians are recognized 

 in the Strata of Purbeck and in Tertiary deposits. 



1 Croc. Harsingsise. 2 Teleosaurus and Steneosaurus. 



3 The Stagonolepis of Elgin. 4 Rhynchosaurus of Shropshire. 



5 In the Paper Coal of the Rhine. 



6 Archegosaurus Labyrinthodon. 



