102 EVOLUTION IN THE PAST 



thoroughly adapted to marine hfe, being provided with 

 powerful paddles. They further resembled snakes as regards 

 the mouth. This organ was capable of considerable dis- 

 tension, as the head bones worked freely upon one another, 

 and the two halves of the lower jaw, joined by elastic liga- 

 ment, could be drawn well apart. The throat, no doubt, was 

 also as with snakes capable of considerable expansion, and the 

 animals were enabled to seize and swallow prey of large size. 

 They possessed also snake-like teeth, pointed and curved 

 backwards. Such teeth were, of course, useless for mastica- 

 tion, but they effectively prevented prey once seized from 

 wriggling out of the mouth. As, however, the mosasaurs 

 were also provided with masticating teeth, it is not hkely 

 that their manner of feeding was entirely snake-like. They 

 attained in some instances to a length of fifty feet ; and ably 

 suppUed the vacancy caused by the early collapse of other 

 sea-going reptiles. 



Some distantly related forms managed to hold their own 

 for a time in the next Period {Champ sosaurus) ; but dohcho- 

 saurs, and mosasaurs, strictly so called, all disappeared in 

 the great reptile " slump " which set in at the close of the 

 Cretaceous. 

 CHELONIANS Tortoises meanwhile had undergone but Uttle change. 

 But however stolid and conservative they may have been as 

 a rule, a spirit of enterprise was in their ranks. A tendency, 

 shown by some river-haunting forms in Jurassic times, to 

 take to marine life had become strengthened in the course of 

 years ; and in late Cretaceous times sea-tortoises or — to 

 speak more correctly — turtles, with limbs modified into 

 true paddles, abounded in some of the shallow seas. Some 

 of these animals seem to have been closely allied to " edible," 

 some to " loggerhead " turtles (Allofleuron, Lytoloma) of our 

 time. 



Other chelonians had become more partial to mother 



earth, and buried themselves in mud more or less completely 



(Trionyx). 



ICHTHYO- Whilst turtles were beginning to swell the ranks of marine 



SAURIANS life, the whale-shaped ichthyosaurs or fish-hzards were 



getting into a parlous condition. They had now lost their 



