285 



• 



species of mosasauroids may have been founded on different ages of the 

 same. 



Fig. 11 represents a humerus accompanying the former specimens, and 

 probably belonging to the same species, if not the same individual. In its 

 form and construction it closely resembles the corresponding bone of C. pro- 

 python. 



The specimen is somewhat crushed, which perhaps to some extent makes 

 it appear proportionately flatter than the humerus of C. propython, described 

 and figured by Professor Cope. 



The length of the bone does not exceed the breadth of its distal extremity, 

 which is the wider one. 



The measurements of the specimen are as follows : 



« Lines. 



Length of humerus at middle . . 35 



Breadth of proximal extremity 2S 



Breadth of distal extremity 35 



Breadth at constricted middle of shaft. 20 



Thickness of head 8£ 



Thickness of distal end 9£ 



Order Lacertilia. (?) 



TYLOSTEUS. 



Tylosteus oknatus. 



The above name has been proposed for a supposed genus of lacertian 

 reptiles, founded on a singular fossil represented in Fig. 14, Plate XIX. 

 The specimen was obtained by Professor Hayden in the "Black Foot" 

 country, at the head of the Missouri River, and was probably derived from 

 the Cretaceous formation. It looks as if it might be an element of the 

 osseous dermal armor of some animal, whether reptile or mammal is by no 

 means certain, though, as before intimated, I suspect the former. 



The specimen is imperfect or broken at the borders. Its inner surface is 

 concave; the outer convex, and ornamented with large mammillary bosses. 

 The latter are about fifteen in number and of different sizes. They are 

 porous and of a less dense character than their shield-like basis. The 

 diameter of the fossil is about 2 inches ; its thickness an inch. 



Accompanying the specimen just described there is an isolated phalanx, 

 represented in Fig. 13. Though suspected to pertain to the same animal, 

 the reference is uncertain. It is a terminal phalanx nearly 2 inches long, 



