ICHTHYOSA URIA 125 



lived in schools, as do the porpoises, each species keeping to its 

 restricted locality and not wandering far. 



The ichthyosaurs began their existence, so far as we now know, 

 about the middle of Triassic times and continued to near the middle 

 of Upper Cretaceous, when they disappeared forever from geological 

 history. As we have seen, however, the earliest forms that we know 

 were true ichthyosaurs in all respects, though more primitive than 

 the later ones, indicating a long previous existence of which we yet 

 have no knowledge. Their remains have been found widely dis- 

 tributed in Triassic rocks of Europe, Spitsbergen, Australia, and 

 North America. During the Jurassic period they lived in great 

 numbers and variety throughout the region that is now Europe. 

 In North America the only marine rocks of this period that we 

 know of have yielded numerous remains. These American ichthyo- 

 saurs were, however, among the most specialized of all ichthyosaurs 

 — the culmination of their development. They were originally 

 named Sauranodon in the belief that they were toothless, but in 

 recent years their teeth, small and numerous, have been discovered. 

 And the genus seems also to be identical with one previously named 

 from the Jurassic of Europe called Ophthalmosaurus. The last 

 known remains of ichthyosaurs have recently been found in the 

 Benton Cretaceous of Wyoming. Scanty remains of ichthyosaurs 

 are also known from AustraHa and New Zealand. Why the 

 ichthyosaurs should have gone out of existence before the plesio- 

 saurs and mosasaurs did, one cannot say; possibly their stock had 

 grown old and feeble. 



