OSTEOLOGY OF CARNIVOROUS DINOSAURS. 



133 



Sacral 1 is opposite the anterior (pubic) peduncle of the ilium, while sacral 3 is 

 largely posterior to the posterior (ischiac) peduncle of the ilium. The centra of 

 sacrals 1,2, and 3 are relatively broad on their ventral aspects, but their surfaces 

 are concave transversely, thus forming a shallow longitudinal groove. This groove 

 is only slightly indicated on the last sacral. One of the striking features of the 

 genus Ornithomimus and perhaps of the family Ornithomimidae is the presence 

 of deep pneumatic cavities on the sides of the centra of sacrals 1, 2, and 3. Such 

 cavities are not present in Anfrodemus, Ceratosawus, and Tyrannosaurus, nor are 

 they mentioned by Lambe in his description of Gorgosaurus. 



The sacral centra are joined to the ilia by short but stout sacral ribs. These 

 ribs do not unite with the centra intervertebrally, but with large buttresses devel- 

 oped on the sides close to the anterior ends. This buttress on the second sacral 

 extends posteriorly somewhat back of the center of the centrum, as showai in 

 figure 67. The neural spines of the sacrals are all joined along the top; transverse 

 thickening alone indicates the point of coalesence of these processes. Between 

 sacrals 2 and 3, and 3 and 4 there are vertical apertures, though in old individuals 

 doubtless these openings would also have filled across with bone to form a con- 

 tinuous plate. Presumably the spinous process of dorsal 13 was also coalesced 

 with the others, though all evidence on this point is missing in the present specimen 



In the articulated skeleton only the processes of the sacrals posterior to the first 

 rise above the superior crests of the ilia, the first being on a level with the superior 

 borders of the ilia, which closely lap it on either side. In the genotype of Sfruthimni- 

 mus the spinous process of sacral 4 is entirely separated from those preceding it, 

 thus differing from Ornithomimus sedens, where it is coalesced at the top, though this 

 represents a difference of degree rather than an important structural change. Ante- 

 roposteriorly the spinous process of sacral 3 measures 100 mm.; sacral 4 measures 

 65 mm.; while the spine of the first caudal measures only 38 mm. in this dimension. 



The length of the four sacral centra is 305 mm., divided as follows: First, 71 mm.; 

 second, 71 mm.; third, 79 mm.; and fourth, 84 mm. 



Caudal vertebrae. — In sequence with the sacrals are the first twelve vertebrae of 

 the caudal series. Their principal measurements are given in the accompanying 



table : 



Measurements in millimeters of the caudal vertebrae of Ornithomimus sedens. 



144035° 



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