OF THE WHITE RIVER BEDS OF MONTANA. 



273 



The Skull — A portion of the base of the skull is preserved, including part of the 

 occiput, the occipital condyles, the basioccipital, the exoccipitals and small portions of 

 the squamosals. 



The occiput was low as in Hyopotamus, which it much resembles in some respects. 

 The occipital condyles and foramen magnum are large. Their lower articulating surfaces 

 are nearly flat medially. They form only a slight angle with the part of the basioccipital 

 anterior to them. Between the paroccipital processes the basioccipital is broad and mod- 

 erately convex. Anterior to this it is broad and quite thin. There is a longitudinal 

 groove on the under side on the median line on the portion between the tympanies. 



The exoccipitals are similar to those of Hyopotamus. They are broad and thin 

 above, convex transversely and concave vertically. They are peculiarly roughened, 

 appearing as if they were made up of several coossified bony plates. The paroccipital 

 processes are both broken off, but they are nearly equilateral triangles in section, nearly 

 on a, level with the inferior surfaces of the occipital condyles. They are directed slightly 

 backward as in Hyopotamus. The exoccipitals are transversely convex posteriorly and 

 concave— except at the top — anteriorly, but they send forward a high ridge or wing 

 which rests against the tympanic; internally. This latter' bone is tightly wedged in 

 between this process, the outer part of the basioccipital and the squamosal. 



The external auditory meatus is small and extends inward horizontally just beneath 

 the lambdoid crest. Aside from this the space between the exoccipital and squamosal is 

 entirely filled with bone. The two processes, the paroccipital and post-glenoid, do not 

 approach each other as in the American species of Hyopotamus. Apparently the 

 arrangement here is more like that in the fragment of skull figured in Kowalevsky's 

 paper on Hyopotamus, in PI. XXXIX, Fig. 5 (1873). The tympanic fills the greater 

 part of the space above referred to. There is a groove extending downward and forward 

 which appears to be a line of bony union, so that the portion anterior to this line may 

 be a triangular exposure of the periotic. The bullae are not preserved, but portions of 

 the petrous were found which show that this bone was very large. One fragment shows 

 a cast of the cochlea. The. first whorl is much larger and is broader than either of the 

 others, which are almost equal in size of whorl, but the third is slightly thicker. The first 

 whorl is .005 m. in diameter and its greatest thickness .0015 m.; third whorl .003 m. in 

 diameter. 



The Atlas and Axis. — Only parts of these two vertebrse are preserved. The atlas 

 has a,n extremely large neural canal, but a considerable portion was occupied by the large, 

 broad and thick odontoid process of the axis. The floor of the canal is convex fore-and-aft, 

 with a median transverse ridge bounding anteriorly the articular surface for the odontoid 

 process. The anterior cotyles, if not confluent below, are almost so and are not separated 



