EPOREODON 63 



occipitals well produced, and occiput medium to high and narrow; malar medium, with zygomata 

 generally lighter posteriorly; bulla; medium to large; mandible medium, symphysis sometimes 

 coossified, ascending ramus wide, and coronoid process low; infraorbital foramen normally 

 above P 3 . 



Dentition: Brachyodont. Superior incisors small, premolars sometimes crowded but usually 

 not, and molars increasing gradually in length. In the upper premolars the posterior crescents are 

 large and the anterior crescents reduced. Normally an anterior intermediate crest is present on the 

 anterior three premolars only, but one may occur on P 4 , as in E. dickinsonensis. E. major relictus 

 has just a trace of an anterior fossette on P\ P 2 and P 3 have either very poorly developed crescents 

 or none at all but have strong anterior, posterior, and median crests, and the posterior basin is divided 

 by a low ridge, extending backward and downward from the median crest. The posterior basin of 

 P 4 is entirely enclosed by the posterior and median crests and the posterior crescent. There is no 

 posterior median ridge but a very small tubercle is present near the center of the basin, and there is 

 a tendency for a pillar-like cusp to develop at the junction of the posterior and median crests. The 

 metastylid of M 3 is large. 



Skeleton: Limbs of medium length and weight, tetradactyle, and with ungues intermediate. 

 The entire skeleton is known from two species, and in the case of a third species the skeleton is 

 nearly complete. 



Discussion: It was Marsh's opinion that Eucrotaphus and Agrioc/iosrus were synonymous, and, 

 therefore, when he erected the genus Eporeodon, he emphasized the presence of large bulla: and the 

 size of the skull, which is larger than that ordinarily seen in Merycoidodon. The absence of a func- 

 tional foramen rotundum and of the pollex are also diagnostic generic characters. At that time 

 (1875), Marsh referred to his new genus Leidy's species Oreodon major and O. bullatus, neither 

 of which was ever placed by Leidy in his genus Eucrotap/ius. 



Eporeodon was one of the most widespread of the merycoidodont genera. It ranged from the 

 Dakotas and Nebraska into Colorado, Montana, and Wyoming and is again found in California and 

 Oregon. The species are very nearly contemporaneous in the Great Plains and in the West Coast 

 areas, ranging from middle Oligocene into lower Miocene. Curiously enough, many of the West 

 Coast species are very similar to the Great Plains forms, as, for example, E. socialis and E. leptacan- 

 thus, which have the same superior molar-premolar index of 0.88 and which are the only two species 

 that are dolichocephalic (skull index of 0.45 for both). E. socialis is a much smaller species, and 

 there are certain other distinguishing characters. E. thurstoni is about the same in size and propor- 

 tions as E. socialis. Again E. bullatus and E. occidentalis are very similarly proportioned, with almost 

 exactly the same skull index, which is nearly in the lower range of brachycephaly, and the molar- 

 premolar indices are not far apart. Likewise, E. major and the four subspecies here included there- 

 with can be matched by E. pacificus, of Oregon, all of them having almost the same skull and 

 dental indices. E. parvus, with its small, almost dolichocephalic skull and high dental index, seems 

 to stand alone, as does E. condoni. E. trigonocephaly is in a class by itself, being the only 

 brachycephalic species. E. longijrons and E. I. perbullatus, of the John Day, fall into a natural 

 group, with the same mesocephalic skull, and they are the only two species in which the lengths of 

 the molar and premolar series are nearly or quite equal. The latter species has a smaller skull, with 

 very much larger bulla;. E. montanus I should consider to be a male, while E. dickinsonensis and 

 possibly E. helena; are the females of the same general group. 



While the number of species and subspecies listed herein appears to be very large, yet there 

 are no two which can be considered sufficiently alike to be conspecific at present. I prefer to regard 

 the subspecies as regional variants. The element of sexual variation does not seem to enter into con- 

 sideration sufficiently to alter the specific characters, for in the majority of species there is enough 

 material apparently to define the male and female distinctions and yet keep the characters of the 

 species clearly diagnostic one from the other. 



I fail to detect any real distinctions which can be used to separate this group into other than 

 this one genus. Certain of the species may have some character which seems not to conform, and 

 yet all of the other characters fall in line with the majority of the generic characters. For example, 



