THE JURASSIC PERIOD. 97 



• 

 eral scheme of division. A division at this point is made particularly 

 infelicitous, so far as the land life is concerned, because the American 

 beds of this stage, which are richest in reptilian remains, the Como or 

 Morrison, have usually been referred to the Jurassic (Pur beck epoch). 

 This reference is now questioned, and they are regarded by many, 

 perhaps by most investigators, as Lower Cretaceous (Wealden epoch), 

 while by some, a portion of the beds in question are regarded as 

 Jurassic and the rest as Lower Cretaceous (Comanchean). This adds 

 grave artificial difficulties to the natural ones. It seems best, there- 

 fore, to follow the leadings of natural evolution, and to consider the 

 reptilian deployment of the Jura-Comanchean land epoch as an essen- 

 tial unit, with some parenthetical guards against erroneous references. 



The Jura-Comanchean development of the land vertebrates. — The 

 anomodonts and some other ancestral reptilian races had followed the 

 stegocephalians into retirement, while other early races lived on in 

 secondary importance. The great feature of the closing Jurassic and 

 opening Comanchean was the marvelous development of the saurian 

 group, which made this the central stage of the " age of reptiles." 



The dominance of the dinosaurs. — The dinosaurs in particular 

 attained remarkable size and diversity, and their dominant species were 

 easily lords of the reptile horde. They deployed not only along the 

 carnivorous line (Theropoda) which had appeared in the Trias, but also 

 on three herbivorous lines (Sauropoda, Ornithopoda , and Stegosauria). 

 Of the carnivores, one of the most typical was Ceratosaurus nasicomis, 

 from the Como beds, whose general aspect, shown in Fig. 371, illus- 

 trates the attitude and proportions of the order. The fore limbs seem 

 to have been used chiefly for seizing and holding prey, and rarely for 

 walking, the animal's pose being facilitated by hollow bones. The head 

 was relatively large, an unusual character for a race among which small 

 heads and diminutive brains were the fashion of the day. Not all 

 the theropods, however, were gigantic ; there were small leaping forms, 

 like Compsognathns, not larger than a rabbit. 



The herbivorous dinosaurs (Stegosauria, Sauropoda, Ornithopoda 1 ) 

 first became known in this system, but their development was so ex- 



1 For monographic treatment see Dinosaurs of North America. O. C. Marsh, 16th 

 Ann. Rept., U. S. Gecl. Surv. The three suborders there recognized are Theropoda, 

 Sauropoda, and Preckntata, Ornithopoda and Stegosauria being regarded as divisions of 

 Predentata. 



