marsh.) CEKATOSAURUS. 161 



Fortunately, in the present specimen of Ceratosaurus, the ilium, 

 ischium, and pubes are firmly coossified, so that their identification 

 and relative positions can not be called in question. The ilia, more- 

 over, were attached to the sacrum, which was in its natural place in the 

 skeleton, and the latter was found nearly in the position in which the 

 animal died. The pelves of Ceratosaurus and of Allosaurus are shown 

 in PI. X. 



The ilium in Ceratosaurus has the same general form as in Megalo- 

 saurus. In most of the other Theropoda, also, this bone has essentially 

 the same shape, and this type may be regarded as characteristic of the 

 order, except in Triassic forms. In Creosaurus the anterior wing is 

 more elevated, and the emargination below it wider, as shown in PL 

 XII, fig. 2, but this may be due in part to the imperfection of the border. 



The ischia in Ceratosaurus are comparatively slender. They project 

 well backward, and for the last half of their length the two are in close 

 apposition. The distal ends are coossified and expanded, as shown in 

 PL X. 



The pubes in Ceratosaurus have their distal ends coossified, as in all 

 Jurassic Theropoda except Hallopus. They project downward and for- 

 ward, and their position in the pelvis is shown in PL X. Seen from the 

 front, they form a Y-shaped figure, which varies in form in different 

 genera. The upper end joins the ilium by a large surface, and the 

 ischium by a smaller attachment. The united distal ends are expanded 

 into an elongate, massive foot, as shown in PL X, which is one of the 

 most peculiar and characteristic parts of the skeleton. The pubes of 

 Coelurus are represented on the same plate. 



The extreme narrowness of the pelvis is oue of the most marked 

 features in this entire group, being in striking contrast to its width 

 in the herbivorous forms found with them. If the Theropoda were 

 viviparous, which some known facts seem to indicate, one difficulty, 

 naturally suggested in the case of a reptile, is removed. 



Another interesting point is the use of the large foot at the lower end 

 of the pubes, which is the most massive part of the skeleton. The only 

 probable use is that it served to support the body in sitting down. 

 That some Triassic dinosaurs sat down on their ischia is proved con- 

 clusively by the 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 group would bring the foot of the pubes to 

 the ground, nearly or quite under the center of gravity of the animal. 

 The legs and ischia would then naturally aid iu keeping the body bal- 

 anced. Possibly this position was assumed habitually by these fero- 

 cious biped reptiles while lying in wait for prey. 



The femur is much curved, and the shaft very hollow. The tibia is 

 shorter than the femur, nearly straight, and has a large cnemial crest. 

 The astragalus is not coosified with the femur, and has a strong ascend- 

 ing process. The fibula is well developed, and nearly straight, its distal 

 16 geol, pt 1 11 



