44 MOSASAURUS. 
The mass of hard blue stone, previously noticed, containing a number of caudal 
vertebre 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 which 
-would endanger the integrity of the specimen. It resembles in general form the 
ulna or fibula of Plesiosawrus, and from its being found with caudal vertebre 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 measurements of the bone are as 
follows :— 
Inches. Lines. 
Extreme length . ; 5 : : : 5 5 c aS 2 
Breadth aboye : ; : : ; " s F 3 cig ay ie 10 
Breadth at the middle . : $ . : : 0 ; 3 yee 4 
Breadth below 5 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 articular surface. The distal articular surface is likewise oval, but 
is concave. ‘The measurements of the specimen are as follows :— 
Lines. 
Length - ° : : , < : 5 4 e c : : BPA 
Breadth of proximal en s - é ; : 5 ° - : Ale! 
Thickness of proximal end . : 5 : : : : c : ; otal | 
Breadth of middle of shaft . ; : 4 : 6 . é : AS ri) 
Thickness of middle of shaft 3 2 A 5 ‘ 3 é S : ee! 
Breadth of distal end . : a 5 A - - - 5 s A . 12 
Thickness of distal end 5 j ‘ - ‘ , 3 5 3 ‘ ae et 
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 
