180 GEOLOGY. 



indicate little advance upon the Jurassic and Wealden forms. The 

 relics arc fragments of hones, jaws, and teeth, all of which seem to 

 represent marsupials or monotremes of small size. They appear to 

 have played a very inconspicuous part in the fauna of the period. 



The Sea Life. 



The sea saurians. — The ichthyosaurs and plesiosaurs which had 

 dominated the Jurassic sea lived on into the Cretaceous, but the ichthyo- 

 saurs almost disappeared soon after the beginning of the period, while 

 the plesiosaurs continued through it, attaining their highest develop- 

 ment and perhaps their greatest size. They had great diversity of 

 form, and were doubtless equally diverse in habit. 



The sea serpents.— The aquatic branch of the scaled saurians 

 (squamata) attained great importance during this period, as veritable 

 sea serpents. The dolichosaurs, long-necked, lizard-like reptiles, were 

 present as early as the Comanchean period, and are not known to 

 have lived after the beginning of the (Upper) Cretaceous. They were 

 the forerunners and perhaps the direct ancestors of the pythonomorphs 

 (mosasaurians) (Fig. 413). The name implies that they were serpent- 



Fig. 413. — A Cretaceous mosasaurid, Platacarpus coryphecus, Cope, restored by Willis- 

 ton, from Upper Cretaceous, Kansas. 



like in form, but this refers chiefly to the elongation and slenderncss 

 of the body. The limbs were retained in less modified forms than 

 those of the ichthyosaurs and plesiosaurs, implying a less complete 

 adaptation to aquatic life. The mosasaurian family flourished in 

 the Cretaceous, and enjoyed a wide distribution, ranging from 

 North and South America to Europe and New Zealand. Their short 

 career seems to have ended with the period, and no direct descendants 

 •are known. The plesiosaurs were notably more specialized than in 

 the early Jurassic (Fig. 413, a). 



The sea turtles. — The first strictly marine turtles appeared in 

 Cretaceous times, and deployed into many and diverse forms. The 

 maximum size of the order was reached in the gigantic Protostega 



