198 GEOLOGICAL SUCCESSION OF ANIMALS. 



and may be considered as the prototypes of the Whales, hav- 

 ing, like them, limbs in the form of oars. The Plesiosaurus 

 , agrees, in many respects, with the Ichthyosaurus, in its 



structure, but is easily distin- 

 guished by its long neck, which 

 resembles somewhat the neck of 

 some of our birds. A still more 

 extraordinary Reptile is the 

 Pterodactylus (Fig. 160), with 

 its long fingers, like those of 

 Fig. 160. a bat, and which is thought 



to have been capable of flying. 



485. It is also in the upper stages of this formation that 

 we first meet with Tortoises. Here also we find impres- 

 sions of several families of insects, (Libellulce, Coleoptera, 

 Ichneumons, <^c.) Finally, in these same stages, the slates 

 of Stonesfield, the first traces of Mammals are found, 

 namely, the jaws and teeth of animals having some re- 

 semblance to the Opossum. 



486. The department of Mollusks is largely represented 

 in all its classes. The peculiar forms of the primary age 

 have almost all disappeared, and are replaced by a much 

 larger quantity of new forms. Of the Brachiopods only one 



bed 



Fig. 161. 



type is very abundant, namely, that of the Terebratula 

 (Fig. 161, a). Among the other Bivalves there are many 

 peculiar forms, as the Goniomya (Z>), and the Trigonia (c). 

 The Gasteropods display a great variety of species, and also 

 the Cephalopods, among which the Ammonites are the 



