462 



MESOZOIC ERA— AGE OF REPTILES. 



Along with these huge animals lived also the smallest Dinosaurs yet 

 known — one of them, Nanosaurus agilis, being about the size of a cat. 



Fig. 745. — Pelvic arch of Morosaurus grandis (after Marsh), seen from in front. One sixteenth 

 natural size: a, fii'st sacral vertebra; b, transverse process of first sacral vertebra; c, trans- 

 verse process of second vertebra; p, fourth or last sacral vertebra; nc, neural canal; il, ilium; 

 is, ischium; pb, pubis. 



The characteristics of these ancient reptiles have been worked out 

 with great skill by Marsh, according to whom the vertebrae of many of 

 them were full of large cavities, so as to make these enormous bones 

 as light as possible. This character reached its highest expression in 

 Coeluria of Marsh (Fig. 743). 



The American Dinosaurs were not only remarkable for size and 

 number but also for their great variety of forms. According to Marsh, 

 some of them were reptile-footed (Sauropoda), some beast-footed 



Brontosaurus excelsis, x T £ B (restored by Marsh). 



(T/ieropoda), some bird-footed (Ornithopoda), and some belonged to a 

 most remarkable family, Stegosauria (plate-covered Saurians), not pre- 

 viously recognized. 



