MERYCHYUS 231 



(1935, p. 169) chooses to retain the species in the genus Merychyus, although recognizing certain 

 similarities to Ticholeptus. His material, identical with Peterson's type, was collected in Goshen 

 County, Wyoming, in strata of lower Harrison age. 



As stated above, M. harrisonensis resembles the Merychyus norm more than that of Ticholeptus. 

 The characters in favor of the former are the skull index of 0.55 ; the shallowness of the skull; the 

 rather slight depth of the malar; the reduced length of the nasal bones, less as compared with that 

 in Ticholeptus; the flat, broad brain case; the moderately shallow infraorbital fossa; the large 

 orbits; the occipital region; the light zygomatic arches; and the general construction of the denti- 

 tion, although P 3 is not so much reduced anteriorly as usual. 



M. harrisonensis resembles Ticholeptus in the greater length of the premolar in respect to the 

 molar series, the index being 0.82. The length is about that of a small Ticholeptus; the face, 

 including the orbits, is more than half the total length of the skull; the facial vacuities are large 

 but still not too large for Merychyus ; the parastyles and metastyles on M 3 are heavy (but this is 

 perhaps owing to the stage of wear) ; and the premolars do not slant backward. 



Several of the characters in which the species resembles Ticholeptus are also like those in Mery- 

 coidodon and Eporeodon and, with others, may be considered primitive. We should expect the 

 lower Miocene forms to be primitive in some characters, and these seem to be as follows: the skull 

 index of 0.55 (Merycoidodon about 0.59); the length of the face and of the tooth row, which is 

 greater than half the skull length and is certainly typical of Merycoidodon; the molar-premolar 

 index of 0.82 (Merycoidodon 0.84), showing the greater relative premolar length; the lack of the 

 usual backward slant to the premolars; the large orbits; the unreduced frontals; the large bulla;, 

 which are close to Eporeodon; the extremely subhypsodont dentition; the heavy styles on M 3 ; and 

 the lack of great reduction in the anterior part of P 3 . 



It is my opinion that this species is the most primitive form of Merychyus, differing very mate- 

 rially from the later forms but not sufficiently to invalidate the reference to this genus. The lack of 

 limb bones is, however, a real deterrent in making any positive classification. 



Merychyus pariogonus Cope 1884 

 PI. XXXIV, figs. 7-9 



Original Reference: Synopsis of the species of Oreodontidre. Proc. Amer. Philos. Soc, XXI, pp. 536. 

 542-545. 



Type Locality: Smith River Valley, Montana. 



Geologic Horizon: Upper Miocene (Deep River). 



Types: Holotype, Cat. No. 8113 A.M.N.H., skull, lacking part anterior to orbits, with left M 2 and M 3 

 complete. Paratype, Cat. No. 8114 A.M.N.H., also back part of skull, with right maxillary and fragments of 

 left maxillary and of mandible, together with other fragments. Collected by J. C. Isaac. 



Specific Characters: The skull is of unusual length, the distance from the posterior of M 3 

 to and including the occipital condyles being 104 mm. This same dimension in other species ranges 

 from 64 mm. (M. delicatus) to 76 mm. (M. a. leptorhynchus), relatively much less than in M. pario- 

 gonus. In all the other species of Merychyus the length of the dentition from the posterior of the 

 canine alveolus to that of M 3 is nearly the same as the length from M 3 to and including the occipital 

 condyles. In M. delicatus, M. a. minimus, and M.siouxensis the dentition is somewhat longer. If 

 this proportion holds true for M. pariogonus, the skull length must have been well over 200 mm., 

 making it the largest species in the genus. But the dimensions of M 2 and M 3 are nearly identical 

 with those of M. a. leptorhynchus, whose skull length is 167 mm. Hence again, if we use the same 

 proportions for the rest of the dentition, we have a much greater emphasis on the postorbital length 

 of skull in this form. The same proportion between dental length, as defined above, and post-molar 

 length is true for the most part in Ticholeptus as well. 



The zygomata are slender, with the maximum expansion in front of the glenoid surface and 

 just back of the postorbital constriction. The posterior elevation is low and angulate rather than 



