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HISTORICAL GEOLOGY 



Armored Dinosaurs (Armored Predentata) 



Hatcher, Marsh, Lull, — The Ceratopsia: Mon. U. S. Geol. Surv., Vol. 49, 1907. 

 Lull, R. S., — The Evolution of Ceratopsia: Seventh Intern. Zool. Congr. Proc, 1910. 

 Lull, R. S., — The Armor of Stegosaurus: Am. Jour. Sci., Vol. 29, 1910, pp. 201-210. 



Crocodiles. — The Triassic ancestral crocodiles have so many 

 characters in common with the primitive reptiles (of the Pelycosaur 

 type, p. 490) and dinosaurs that the order to which they belong is 

 determined with difficulty. Among the changes that the crocodiles 

 underwent during the Mesozoic, the following may be mentioned. 

 (1) The vertebrae were biconcave (amphiccelous) in the Triassic, 

 Jurassic, and most of the Cretaceous, as in fish, and not concave in 

 front and convex behind (proccelous) as in modern genera. (2) The 

 older crocodiles had the opening of the nasal passages into the mouth 

 placed far forward, whereas living crocodiles have them placed in 

 the extreme back of the mouth. This change in position is of advan- 

 tage in that it makes it possible for the animal to breathe while it is 

 drowning its prey. The early Mesozoic crocodiles were probably 

 obliged to go to the land to devour their food. The marine crocodiles 

 were doubtless descended from a group that lived in rivers, and these, 

 .in turn, from terrestrial or amphibious ancestors, although the 

 earliest known crocodiles are marine. 



Shortly before the close of the Jurassic, a side branch (Thalat- 

 tosuchia) appeared, which were thoroughly adapted to a marine 

 existence. They were covered with a bare skin, without scales, and 

 the tail ended in a long fin. The fore limbs were paddle-like, while 

 the hind limbs were less modified, probably because of the necessity 

 of visiting the shore for egg-laying. After a brief existence, this 

 family disappeared. 



The crocodiles underwent a marked change early in the Cre- 

 taceous, at which time the more modern crocodiles and gavials were 

 developed. 



Marine Reptiles. — One of the most significant features of reptilian 

 evolution is the way in which the reptiles, after they had become 

 adapted to land life, were enabled by their superior organization and 

 greater activity as tftV-breathing animals, to re-invade the sea re- 

 peatedly and successfully. Hardly had the reptiles become well- 

 established upon the land, before some took to the water and became 

 perfectly adapted to a marine existence. Members not only of one 

 but of several classes of reptiles were so modified. It is not remark- 



