OSTEOLOGY OF THE ARMOBED DINOSATJEIA. 



45 



not require the forcible mastication of that of Iguanodon or more especially of the 

 late Cretaceous Trachodon, in which the 

 powerful dental battery of 2,000 teeth reached 

 its highest perfection." 



Vertebral Column. 



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13 H 



2 K 



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The vertebral formula in Stegosaurus, as 

 nearly as can be determined from available 

 material, is as follows: C, 10(8); D, 17(?); 

 S, 3; C, 45 to 47. There are no true lumbars. 

 Some uncertainty exists regarding the exact 

 point of separation between the cervical and 

 dorsal region. Although slightly disarranged, 

 the type of S. stenops, No. 4934, shows the 

 entire number of vertebrae in front of the 

 sacrum (including the dorso-sacral) as being 

 27. Marsh in his first restoration of S. 

 ungulatus shows 26 presacrals, 4 sacrals, and 

 44 caudals. 



In regard to the caudal series it would 

 appear that the tail of Stegosaurus, as in other 

 dinosaurs, is not made up of a constant num- 

 ber of vertebrae but within limits varies with 

 the individual. 



Atlas. — The atlas in Stegosaurus consists 

 of the usual four pieces, the intercentrum, 

 neuracentra, and odontoid process. It resem- 

 bles very closely the corresponding element in 

 Camptosaurus. 



The intercentrum, as shown by specimen 

 No. 4934, is a subcrescentic block of bone, the 

 concave side being uppermost and with the 

 greatest diameter transverse. The upper pos- 

 terior part of the transversely concave surface 

 receives the odontoid while the anterior part 

 of this surface is more deeply excavated, form- 

 ing the lower portion of the cup for articulation 

 with the occipital condyle of the skull. 



On either side of this concave portion are 

 the articular surfaces on which the pedicels of 

 the neuracentra rest. Viewed from behind 

 this element presents a nearly straight ver- 

 tical face for articulation with the axis. On 

 either side of the inferior surface are well- 

 developed facets for the articulation of the single-headed cervical ribs shown in 

 figure 16,?"'. 



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