216 the merycoidodontim: 



the position of the infraorbital foramen, and a general similarity in size and construction of the 

 teeth. 



The chief characters in which Oreodontoides differs from Merychyus are the much lighter 

 malar below the orbit, the much greater postorbital constriction diameter (25 per cent larger than in 

 M. curtus and M. a. minimus, forms which are most nearly comparable in size), the greater diameter 

 of the brain case, the lack of any trace of a facial or frontal vacuity, the lack of sagittal crest, the 

 shorter nasals, and the more uniform depth of the ramus below the tooth row. The teeth compare 

 most closely to those in M. curtus, but with the following differences in Oreodontoides: the pre- 

 molars have no backward slant, are considerably less reduced anteriorly, and are narrower, with the 

 exception of P 1 . P 3 is longer and P 4 shorter than in M. curtus, while M 1 is nearly 3 mm. shorter 

 and 1 mm. narrower, M 2 proportionally about 2 mm. smaller, and M 3 about the same in size. The 

 molar-premolar index is 0.95, as a result of the nearly equal length of each series — an index that is 

 considerably higher than any in the Merychyus species. 



It is true that many of these dental characters may be considered as primitive and leading toward 

 Merychyus, but the construction of the superior premolars is nowhere nearly so advanced in the 

 anterior shortening of the premolars as in contemporaneous merychyids. The inferior premolars are 

 not reduced in anteroposterior length, and as a consequence the first three overlap nearly half of their 

 diameter. Again comparing with M. curtus, Pi, P 2 , and P 3 are considerably larger, while P 4 is of 

 almost exactly the same size. 



In the general shape of the skull, Oreodontoides resembles Merycoldes cursor in the wide 

 frontals, great diameter of the postorbital constriction, wide and flattened brain case, low skull, lack 

 of vacuities, infraorbital foramen above the interval between P 3 and P 4 , deep lacrimal pit, large orbits, 

 presence of bullse, general shape of muzzle, and brachyodont dentition. 



Oreodontoides differs from Merycoldes in being much smaller in skull length (about one-third 

 less than M. cursor, which is the smallest species in that genus) ; in having the nasal bones more 

 reduced, the malar relatively smaller, and the anterior ridge much less prominent, the palatonarial 

 border farther forward, the posterior part of the skull more depressed} in the lack of union of the 

 temporal ridges; and in being more brachycephalic. The dental series show distinctions from 

 Merycoldes in that the premolar length is relatively considerably greater, the molar-premolar indi- 

 vidual lengths are proportionally similar except that P 2 and P 3 are somewhat greater in the Oregon 

 species, the anterior part of P 3 is not nearly so much reduced, the styles on the molars are thinner and 

 straighter, P 2 and P 3 overlap and are obliquely placed, while all the lower premolars are somewhat 

 differently constructed and proportioned. 



While this Oregon form has certain characters similar to those in both Merychyus and Mery- 

 coldes, I believe that the disparities are of sufficient importance to rule it out of both genera, although 

 it is closer to the former, in my opinion. I consider it a primitive merychyid. It seems to me that it 

 represents an offshoot from the Merychyus stem stock which, so far as we now know, developed its 

 own peculiar features in this one West Coast locality. At any rate, it is the nearest approach to 

 Merychyus in that area that has been described to date. 



Genus MERYCHYUS Leidy 1858 



Table 1 2 



Original Reference: Notice of remains of extinct Vertebrata. Proc. Acad. Nat. Sci. Phila., X, p. 25. 



Genotype: M. elegans Leidy. 



Genocotypes: Cat. Nos. 120 and 121 U.S.N.M., halves of upper and lower jaws with teeth. 



Distinguishing Characters: Skull medium sized (140 mm.-200 mm. long), mesocephalic 

 (averaging an index of 0.57) and low, the highest point being at the postorbital constriction; facial 

 vacuities (none in M. dellcatus) small to medium sized; nasals somewhat shortened, not extending 



