THE MAMMALIA OF THE DEEP RIVEK BEDS. 129 



like that in Eporeodon that no detailed description of it will be necessary, and it will 

 therefore suffice to mention the points of difference between the two genera. The 

 general proportions of the skull, length of face and cranium, size and position of the 

 orbits, etc., are very similar in both ; it is only when we come to compare the details 

 that differences become apparent. In Mesoreodon the anterior aspect of the premax- 

 illaries is slightly broader and more flattened, and the two bones are more closely 

 applied together and the symphyseal portion is more thickened. These changes are 

 very slight, but they are not unimportant, since they are in the direction of the 

 curious ankylosed premaxillai'ies of Merycliyus. The maxillary sinuses are enlarged, 

 which gives to the face a slightly swollen appearance. As in the older genera of the 

 family, there appears to be a sexual difference in the lachrymal depression, the depth 

 of which varies with the size of the canines, indicating that it was better marked in 

 the males, but it is never so deep as in the males of Eporeodon. The frontal sinuses 

 are more inflated than in the John Day genus, which gives to the forehead somewhat 

 the same vaulted appearance as in Merycliyus, but to a less degree; the nasal pro- 

 cesses of the frontals are unusually long. 



The zygomatic process of the squamosal is intermediate in character between 

 that of Eporeodon and that of Merycliyus zygomaticus ; it is very widely expanded at 

 the base, both transversely and antero-posteriorly, and its outer border is quite 

 strongly raised, thickened and rugose, more so than in the former, less so than in the 

 latter. The postglenoid process resembles that in the John Day form in being low, 

 broad and very massive. The tympanic bullae vary in size, being in some specimens 

 much more prominent and inflated than in others. So far as the material in hand 

 goes, it appears to indicate that the bullae were more largely inflated in the male than 

 in the female, but a much larger series of skulls 'will be required to definitely deter- 

 mine whether this is really a sexual character or not. The occiput is peculiar, and 

 in its upper portion very similar to that of Eporeodon, the angles being extended into 

 a pair of large wing-like processes as in that genus and in some species of Oreodon 

 as well. These prominent processes are separated by a deep concavity; beneath this 

 the surface is transversely very convex, prominent in the median line, and with deep 

 grooves or narrow fossae at the sutures between the squamosals and the exoccipitals. 

 The wide transverse expansion of the latter elements makes the base of the occiput 

 very broad. The paroccipitals are likewise broad at the base and closely applied to 

 the bullae, but the distal portion is slender and tapering. In these respects the 

 inferior portion of the occiput is intermediate in structure and appearance between 

 that of Eporeodon and that of Merycliyus zygomaticus. As in the latter, the condyles 

 project more posteriorly than in the John Day form. 



