THE MAMMALIA OF THE DEEP RIVER BEDS. 151 



Merycochceuus montanus Cope. 



Proceedings Amer. Philos. Soc, Vol. XXI, p. 531. 



This species is one of the most abundant and characteristic forms of the upper 

 beds and is represented in the collection by specimens which include nearly all parts 

 of the skeleton, the only important structures which are absent being the scapula 

 and pelvis. 



The skull and dentition of this genus are well known, and for this species in 

 particular have been very fully described by Cope. It will suffice, therefore, for our 

 present purpose to give a brief summary of the more characteristic features of these 

 structures, with especial attention paid to the numerous variations displayed by the 

 various specimens. 



As in the other members of the genus, the face is bent downward upon the basi- 

 cranial axis and is much more elongate than in the other genera of the family, all of 

 which are characterized by short faces, and in some this shortening is extreme. The 

 brain-case, on the other hand, is relatively short and well rounded. The orbit is 

 small and situated high in the face. The occiput is high and narrow above, though 

 broader than in the other species of the genus in which this region of the skull is 

 known, and becomes very wide at the base. Above the foramen magnum is a nar- 

 row but strong median convexity, which is bounded on each side by a deep fossa and 

 which passes superiorly into the shallow concavity which is enclosed between the 

 wing-like processes of the supraoccipital and parietals. The latter processes project 

 much less strongly backward and more transversely than in the older genera of oreo- 

 donts or than in the John Day species of MerycocJmrus. The occipital condyles do 

 not project very strongly behind the plane of the paroccipital processes, though in 

 this respect there appears to be considerable individual variation. The foramen 

 magnum is unusually high and narrow. The paroccipital processes are very broad 

 at the base, but long, tapering, slender, and antero-posteriorly compressed in the free 

 portion. The tympanic bulla? are large, especially from before backward, extending 

 anteriorly beyond the line of the postglenoid processes and nearly to that of the 

 glenoid articular surfaces; between the tympanies the basioccipital is narrow and 

 compressed and has a strong inferior keel. The squamosal forms most of the side 

 wall of the cranium and sends off a massive zygomatic process, which, however, is 

 not so heavy as in the John Day species, M. macrostegus. The different specimens 

 in the collection exhibit considerable variation in regard to the weight of the zygo- 

 matic process, which does not appear to be of a sexual character, for the differences 

 are not correlated with the size of the canine teeth. In the type of M. montamis, 



A. P. S. VOL. XVIII. T. 



