44 MOSASAURUS. 



The mass of hard bhie stone, previously noticed, containing a number of caudal 

 vertebriB in a row, obtained by Dr. Hayden on the Yellowstone River, also contains 

 a broad bone, represented in Fig. 12, Plate XVII, together with several small 

 bones, like that represented in Fig. 13, of the same Plate, all of which appear to 

 have belonged to the same skeleton. 



The broad bone is yet partially imbedded in its hard matrix, the removal of Avliich 

 would endanger the integrity of the specimen. It resembles in general form the 

 rdna or fibula of Plesiosaurus, and from its being found with caudal vertebra? might 

 be suspected to be the latter bone. It is not, however, unlikely that it may prove 

 to be a tarsal bone, one of which it likewise resembles in the foot of Plesiosaurus. It 

 is about as broad as it is long, and the exposed surface exhibits a radiate ossific 

 structure. Internally the upper part of this surface is transversely convex, but is 

 slightly depressed below ; externally it is concave. The upper border is convex in 

 the direction of the breadth of the bone. The inner border is longitudinally con- 

 cave, transversely convex, thick, and smooth. The outer border is thin, emarginate, 

 and presents an irregular pitted appearance as if it had been covered with cartilage. 

 The lower border at its inner third forms an obtuse angle, is thick, and subdivided 

 into a pair of concave articular facets. The measiu'cmeuts of the bone are as 

 follows : — 



Inches. Lines. 



Extreme length 3 2 



Breadth above ........... 2 10 



Breadth at the middle .......... 2 4 



Breadth below 3 2 



Thickness at the lower border 1 2 



Thickness at the outer border ........ 4 



The small bones above alluded to appear to be metatarsals and phalanges ; and 

 they closely resemble the corresponding bones of the Plesiosaurus. One of the 

 best specimens, represented in Fig. 13, Plate XVII, has a quadrate shaft strongly 

 expanded at the extremities. The proximal end is the larger, and presents a trans- 

 versely oval, flat articidar surface. The distal articular surface is likewise oval, but 

 is concave. The measurements of the specimen are as follows : — 



Lines. 



Length 22 



Breadth of proximal end 14 



Thickness of proximal end Y 



Breadth of middle of shaft ........... 5 



Thickness of middle of shaft 4 



Breadth of distal end 12 



Thickness of distal end ........... 7 



Two additional specimens, partially imbedded in their matrix, present very nearly 

 the same form and size as that just described. 



Other bones of the limbs, which may, with the same probability as the preceding, 

 be referred to Mosasaurus, I have not had the opportunity of examining. 



An isolated bone, somewhat crushed in appearance, belonging to the cabinet of 

 Prof James Hall, and obtained by Messrs. Meek and Hayden, from the Cretaceous 



