JURASSIC VKIITEI'.UATE FOSSILS. 



505 



Tlic cervical vertebrae of Ceratosaurus differ in type from those in any 

 other known reptiles. With the exception of the atlas, nil are strongly opis- 

 thocoelian, the cup on the posterior end of each centrum being unusually 

 deep. In place of ;m equally developed ball on the anterior end, there is a 

 perfectly flat surface. The size of the latter is such that it can he inserted 

 only a short distance in the adjoining cup, and this distance is accurately 

 marked on tin- centrum by a narrow articular border, just hack of the flat 

 anterior face. This peculiar articulation leaves more than three-fourths of 

 the cup unoccupied by the succeeding vertebra. 



Fig. 61. — Pelvis of -i UosaurvsfragilU Marsh ; side 

 a, acetabulum ; il, ilium . tv, ischium ; p, pubis. 



i in i, ii i ine-twelfth Datura] isize. 



The pubes have their distal ends coossified, and expand into an elon- 

 gate, massive font, which is one of the most characteristic parts of the skele- 

 ton. It is probable that this foot in connection with the distal ends of the 

 ischia served to support the body in sitting down. That some Triassic 

 dinosaurs sat down on their ischia is proved conclusively bythe impressions 

 in the Connecticut River sandstone In such cases the leg was bent SO as 

 to bring the heel to the ground. The same action in the present reptile 



