38 MOSASAURUS. 



Little ISIoreau River, near Fort Pierre. The length of the more perfect is fifteen 

 lines, tlie lieight twenty lines, and the width twenty-one lines. 



4. A candal body and a posterior dorsal body, from the forks of Cheyenne River. 

 The former possesses rudimental transverse processes projecting from the middle of 

 the sides, and measures eighteen lines in length, twenty-one in height, and twenty 

 in width. The latter belonged to a very small species, comparatively, or to a very 

 young individual of the larger species, but presents no indication of sutural attach- 

 ment of parts. It measures fifteen lines in length, sixteen lines high, and nineteen 

 lines wide. 



5. A mutilated posterior dorsal body, and a mutilated anterior caudal body, from 

 Little Moreau River, near Fort Pierre. The former had robust, transverse pro- 

 cesses projecting from the middle of the sides of the body anteriorly. Its length 

 is thirty-six lines, the height of its posterior convexity thirty-four lines, and its 

 Avidth thirty-three lines. The caudal had no chc\Ton bones, but strong transverse 

 processes projecting from the lower part of the sides of the body. The articular 

 extremities in outline are triangidar with strongly rounded angles. The length of 

 the body is twenty-eight lines, the height of the posterior convexity about twenty- 

 six Hnes, and the width below twenty-nine lines. 



The vertebrae of Mosasaurus, previously mentioned as having been received from 

 Dr. Spillman, Avho discovered them near Columbus, Mississippi, consist of two cer • 

 vicals and the fragment of a dorsal. 



The cervical vertebrae, represented in Figs. 4, 5, 6, Plate VII, have their body 

 twenty-nine lines long, with a transversely oval posterior convexity twenty lines 

 wide and sixteen lines high. The spinous process, long, strong, and laterally flat- 

 tened, measures about two and a half inches along its anterior border. The spinal 

 canal is about seven lines high and six lines wide. The transverse processes are of 

 robust proportions and remarkable form. Springing from the conjunction of the 

 vertebral arch and body, they form a rectangular protuberance, at first descending 

 upon the side of the latter and then turnmg forward at a right angle to its anterior 

 border. Their upper extremity is thick ; and they narrow in their descent and 

 anterior extension. Their free extremity presents an L shaped articular surface 

 for a cervical rib. The hypapophysis is a strong process projecting doAvnward from 

 the back portion of the under side of the body ; springing from a carina-like base 

 it ends in an ovoid, truncated extremity, as seen in Fig. 4, Plate VII. 



The fragment of a dorsal vertebra, represented in Fig. 7, Plate VII, has the 

 body thirty lines in length, and in its details resembles the cervicals just described, 

 except that its hypapophysis is a mere rudiment, indicating the position of the 

 vertebra to be the next succeeding those anterior dorsals which possess a more 

 distinctly developed process of the same character. 



All the vertebrae Avhich I have described or indicated, and indeed all the speci- 

 mens I have seen, are apparently complete in their development, that is to say, 

 none of them exhibit marks of original separation of the composite elements. 

 The union of these in reptiles, especially the complete coossification of the verte- 

 bral arch and body, indicates maturity in the skeleton. Never having seen Mosa- 

 sauroid vertebrae exhibiting certain signs of immaturity I have associated this 



