88 



GEOLOGY. 



haps the shelless cephalopods, whose advent has been noticed. The 

 remains of 200 belemnites have been found in a single stomach. There 



Fig. 364. — A Jurassic ccelacanth, Undina gulo, a crossopterygian, about one-seventh 

 natural size; the outline of the air-bladder is shown just back of the gills and under 

 the axis. (Restored by A. Smith Woodward.) 



were small as well as large forms of ichthyosaurs, some of the latter 

 reaching 30 feet or more in length. 



Descended from a quite different stock, the plesiosaurs adapted 

 themselves to sea life in their own fashion (Fig. 367). Instead of 

 acquiring the flowing lines of a fish, the body took on a form more 



Fig. 365. — °hotograph of Ichthyosaurus quadriscissus Quenstedt, showing outline 

 of paddle j, fins, and body, as well as the skeleton. From the Lias of Wi'irtem- 

 berg, from specimen in Carnegie Museum. (Per kindness of Director Holland.) 



like that of a turtle, while the neck was very elongate, giving rise to 

 the epigrammatic description "the body of a turtle strung on a snake." 

 The earlier representatives, the nothosaurs, were but partially aquatic, 

 while the true plesiosaurs were wholly so. The limbs of these latter 

 were developed into paddles rather than fins, and were sometimes 

 more than six feet long. Locomotion seems to have been chiefly 



