scott: toxodonta OF THE SANTA CRUZ beds. 
135 
part of the occiput proper ; its dorsal border is a heavy and prominent 
crest, which is relatively short from side to side, as the supra-occipital does 
not expand greatly above the constriction of the exoccipitals. In the young 
skull the exposed surface of the supra-occipital is nearly plane, but in the 
adult animal it becomes quite deeply concave, in which stage the bone is 
very thick and massive, with cancellous interior, but without any distinct 
sinus. 
The basisphenoid is also long, passing forward into the posterior nares 
and tapering anteriorly ; near the middle of its course, on each side, is a 
massive projection for the attachment of the alisphenoid. Very little of 
the presphenoid is visible, as it is mostly covered and concealed by the 
pterygoids. The alisphenoid is very narrow antero-posteriorly, but has a 
very considerable dorso-ventral extent and sends down a long, narrow 
descending process, or pterygoid plate, which ends distally in a prominent 
knob. The limits of the orbitosphenoid are not distinctly shown in any 
skull that I have examined. 
The parietals are relatively small and especially narrow, forming but 
little of the side-walls of the cranium ; posteriorly, at the junction with 
the supraoccipital, they are very thick and massive, thinning forward, and 
have a diploetic structure, but I have seen no sinus in them. For most 
of their length, they carry the sagittal crest, but anteriorly the ends 
diverge, to receive between them the posterior extensions of the frontals. 
The cranial cavity, as seen in a longitudinally bisected skull, is enclosed 
almost entirely by the occipitals, sphenoids, parietals and squamosals, the 
frontals being but little involved. The cerebral fossa is small and ap- 
parently does not extend back over the cerebellar fossa ; the olfactory 
fossae are surprisingly small in view of the otherwise primitive character 
of the brain. Nothing can be learned from the material at my disposal 
regarding the number and course of the cerebral convolutions. 
The squamosals are very large, forming much the greater portion of 
the side-walls of the cranium and even part of the floor, where the bone 
is incurved internal to the glenoid cavity. This cavity is a saddle-shaped 
surface, narrow and convex antero-posteriorly, broad and slightly con- 
cave transversely; its outer end forms a prominent protuberance, much 
as in the horse, but larger and more conspicuous. Separated from the 
cavity by a notch, which is very broad externally and narrows inward, is 
the large and swollen-looking postglenoid process, which is filled with 
