230 GEOLOGICAL SUCCESSION OF ANIMALS. 



then), limbs in the form of oars. The Plesiosaurus (b) 



agrees, in many respects, with 

 the Ichthyosaurus, in its struc- 

 ture, but is easily distinguished 

 by its long neck, which resem- 

 bles somewhat the neck of 

 some of our birds. -A still 

 more extraordinary Reptile is 

 Fig. 160. the Pterodactylus, (Fig. 160,) 



with its long fingers, like those of 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 impressions 

 of several families of insects, (Libellula, Coleoptera, Ichneu- 

 mons, Sfc.] 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 resemblance 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 

 greater variety of new forms. Of the Brachiopods only one 



b c d 



Fig. 161. 



type is very abundant, namely, the Terebr alula, (Fig. 161, a.) 

 Among the other Bivalves there are many peculiar forms, as 

 the Goniomya (b) and the Trigonia, (c.) The Gasteropods 

 display a great variety of species, and also the Cephalopods, 

 among which the Ammonites are the most prominent, (d.) 

 There are also found, for the first time, numerous represen- 

 tatives of the Cuttle-fishes, under the form of Belemnites, 



