46 THE MF.RYCOIDODONTID.T. 



It has been suggested that M. gracilis is the female of M. culbertsonii, but the differences 

 between the two are very constant and many of them are too great for any such close connection. 

 For example, the bullse in the former are very large in relation to the skull, which is much smaller 

 than that of the latter species. The posterior part of the nasals is truncated in the smaller species 

 and acute in the larger, and there are many other important and constant differences which indicate 

 to me two well defined species. 



Bump and Loomis (1930) made a quantitative study of a series of skulls representing the four 

 species, M. affinis, culbertsonii, gracilis, and periculorum. Their results showed that M. gracilis and 

 M. periculorum each formed a very uniform assemblage, while each of the other two species varied 

 materially. No apparent sex differentiation in size was noted. The skull variation in 1\I. affinis and 

 M. culbertsonii concerns not only the length but also the width, and thus narrow, medium, and wide 

 skulls are produced. 



The authors offer the following in explanation of the variation. 



The range or variation in M. affinis and M. culbertsonii is a large one, large enough to cause speculation as 

 to its cause. It would seem to indicate that stress was being put on these species, probably in that the environ- 

 ment was changing, and the species were trying to adjust themselves to changing conditions. Some of the indi- 

 viduals seem to be dwarfed, others normal, and a few over large ; and some of the other herbivores of this same 

 time likewise show wide variations, such as Mesohippus and Poebrotherhun. 



This genus is found in lower Canada, as known from the work of Cope (1891) and Lambe 

 (1908). 



It is perfectly apparent that Merycoidodon has priority over Oreodon as a generic name, 

 although Leidy preferred the latter and this name has been used by subsequent writers until fairly 

 recently. From this the name "oreodonts" to designate the entire family has come into universal 

 use, and there is no objection to its continuance, although "merycoidodonts" is a more proper term 

 but more cumbersome. 



Etymology: Merycoidodon (ruminant + form + tooth); Oreodon (mountain + tooth); 

 Cotylops (cup, socket + face, in allusion to large antorbital fossa). 



Species: 



M. affinis (Leidy) 1869. 



M. culbertsonii Leidy 1848. Genotype. 



M. culbertsonii periculorum (Cope) 1884. 



M. gracilis (Leidy) 1851. 



M.macrorh inus (Douglass) 1901 (1903). 



M. platycephalus Thorpe 1921. 



Merycoidodon affinis (Leidy) 1869 

 PI. I, fig. 12 



Original Reference: The extinct mammalian fauna of Dakota and Nebraska, including an account of 

 some allied forms from other localities, together with a synopsis of the mammalian remains of North America. 

 Jour. Acad. Nat. Sci. Phila. (2), VII, p. 96 {Oreodon affinis). 



Type Localities: Niobrara River (HT); near Scotts Bluff and on the Warbonnet Ranch, Sioux 

 County (PLT), Nebraska. Referred specimens from Colorado and Wyoming. 



Geologic Horizon: Lower and middle Oligocene (Chadron and lower Brule). 



Types: Holotype, Cat. No. 106SU A.N.S.P., facial part of skull, in advance of postorbital constriction 

 about as far forward as first premolar. Plesiotypes, Cat. No. 12227 Y.P.M., skull, lacking mainly condyles, part 

 of posterior of cranium, zygomatic arches, and incisors, but with right ramus with teeth, two vertebra?, part of 

 pelvis, calcaneum, and some bone fragments; Cat. No. 12748 Y.P.M., skull somewhat laterally crushed. 



