Peterson: The Osteology of Promerycochcerus. 199 



Although this species cannot for the present be separated generically 

 from the genus Promerycochcerus there are a number of characteristic 

 features which at once separate it from the earHer forms P. chelydra 

 and P. carrikeri. P. hollandi is also more nearly related to the earlier 

 rj'pes. Two of the more significant specializations of P. vantasselensis 

 are included as specific characters, viz: the lengthening of the space 

 for the molars and the shortening of the space for the premolars as 

 well as the shortening of the nasals. These characters suggest the 

 condition in Merycochceriis found in a later horizon, but in the latter 

 genus the cranium is shorter, the brain case is larger, the occiput is 

 broader, the facial region entirely changed; i.e., the heavy inflated 

 longitudinal ridge separating the side of the face into two fossae is 

 absent, the infra-orbital foramen is further back, and the nasals are 

 very considerably shorter and constricted in the middle antero- 

 posterior region. These are characters which undoubtedly indicate 

 an earlier origin (Oligocene) than those indicated in the species under 

 description (compare fig. I on Plate XL with fig. 2 on Plate XLII). 



A more minute study of the remains of this species reveals a great 

 many characters which differ from those in P. carrikeri and other 

 species of this genus. Some of these differences are of importance, 

 and a detailed comparative description is therefore in order. 



the skull. 

 (Plate XL). 



The skull is not so broad proportionally as in P. chelydra and P. car- 

 rikeri, but slightly broader than In P. hollandi or P. temporalis. The 

 high occiput is similarly constructed though possibly somewhat more 

 overhanging than in these species, but the occipital plate is approxi- 

 mately the same. The condyles are quite sessile and point more 

 downward than in P. carrikeri, more closely resembling those in 

 P. hollandi, resulting in a more curved basicranial axis in the present 

 species and in P. hollandi. 



The size of the brain-case is perceptibly larger than in the earlier 

 species, a fact which is especially noticeable in the type, while in the 

 paratype. No. 1230, the brain-case has received lateral crushing. The 

 sagittal crest is generally sharper than in P. carrikeri, and in this 

 respect is also similar to P. hollandi, but the crest has a greater con- 

 vexity fore-and-aft than in the latter species. The zygomatic arch 



