106 REPTILES 



their contemporaries the ichthyosaurs, appear to have more or 

 less completely died out, although a few may have survived 

 into the early part of the succeeding Tertiary period. 



Not till the Upper Cretaceous did the great snake-like 

 swimming marine reptiles which may be popularly called sea- 

 serpents or mosasaurs but are technically termed Pythono- 

 morpha, make their appearance on the scene. They form 

 a subordinal group of the order Squamata of equal rank with 

 the Lacertilia and Ophidia. The body was greatly elongated, 

 with paddles widely sundered from one another, and there 

 does not appear to have been a back-fin. As in snakes, the 

 two halves of the lower jaw were movably united by ligament 

 in front ; there were no bony plates in the eye ; and the skin 

 was probably naked. The typical Mosasaurns (so called from 

 the Latin name of the river Meuse, on the banks of which its 

 remains were first discovered) had a skull of about four feet 

 long, and attained a total length of something like five-and- 

 twenty feet. The giant of the group — a real " sea-serpent " — 

 seems, however, to have been a New Zealand representative of 

 the genus Ltodon, whose length has been estimated at no less 

 than ioo feet. The Python omorpha may have been derived 

 from lizards more or less nearly related to the monitors (Vara- 

 iiidai), some of the extinct representatives of which attained 

 hugh dimensions. 



Yet another extinct group of marine reptiles allied to the 

 lizards is represented by the Dolichosauria, which form a 

 second suborder of Squamata, characterised by the paddle-like 

 form of the limbs being less pronounced than in the Pythono- 

 morpha and by the bony union of the two halves of the lower 

 jaw. These reptiles likewise came into existence only with the 

 Chalk and disappeared with the Tertiary. The typical Dolicho- 

 saurus of the English Chalk was a reptile of about a yard in 

 length, as was also Acteosaurus of I stria, in which the front- 

 paddles are known to have been decidedly smaller than the 

 hind-pair, while the tail was long. 



The sea-snakes, forming the subfamily Hydrophiince of the 

 Colubridce, afford an instance of what may be called a double 

 adaptive modification, for they are evidently descended from 

 ordinary snakes, which have been evolved from a four-limbed 

 group by the excessive elongation of the body and the loss of 



