MERYCOIDODON 55 



M. culbertsonii, but, in the absence of complete skulls, I hesitate to state definitely that this species 

 occurs in the John Day Basin, although I rather believe that it does. 



This species is reported by Cope and Lambe (Cat. Nos. 6272-6274 N.M.C.) from the lower 

 Oligocene (Cypress Hills) of southern Canada, one of the extremely rare instances where a member 

 of this family has been found outside of the United States. 



The South Dakota School of Mines, at Rapid City, has the skull and jaws (Cat. No. 311) of a 

 very young individual of this species with all of the sutures open. Professor W. B. Scott very kindly 

 gave me a photograph of this most interesting specimen, and Mr. Bruce Horsfall most generously 

 made the illustrations, shown in PL II, figs. 3-4. The skull is wide and short, the brain case and 

 frontals are very wide, and the nasal bones are very short. The interparietal is large, and the supra- 

 occipital has no crest. The premaxillaries are relatively proportional, the zygomatic arch is very 

 light, the orbit very large, and the external auditory meatus unusually large. There is no sagittal 

 crest, and there are no temporal ridges, and the superior contour is arched above the postorbital 

 constriction. The chin is straighter than in the adult condition and has a symphysiodental angle of 

 50°. The lower incisors are much longer than the superior. The canines are like pegs. The 

 inferior true canine has the shape and size of the lower incisors, and all are nearly uniform in shape 

 and especially in size. There are but three teeth behind the canine in both jaws. 



This skull belongs to one of the skeletons of a pair of fcetal twins, found lying within the 

 pelvic girdle of an excellently preserved skeleton of M. . culbertsonii. The slab containing the three 

 skeletons was collected on Cain Creek, about two miles north of Imlay, South Dakota, and was first 

 described by the late Professor C. C. O'Harra (1930). 



In regard to the age at death of 210 individuals of this species in the Marsh Collection — 20.9 

 per cent were young, 65.7 per cent were fully adult, 10 per cent had just passed middle age, and 

 3.3 per cent had reached old age. 



In comparison with the figures resulting from a similar study of the John Day eporeodonts, we 

 find a remarkable agreement in the percentage of those that died in the prime of life, 65 per cent in 

 both cases. In the other age classes, the figures seem to indicate that the infant mortality in the 

 Great Plains area was greater than in the John Day Basin and that more individuals reached an 

 older age in that basin. 



These two groups are widely separated in time and in geographic locality, and undoubtedly 

 their habitats differed considerably. 



In the Columbia University Museum are two skulls, one labeled M. culbertsonii and the other 

 M. gracilis, and both supposedly came from Cowhouse Creek, Coryell County, Texas. In discus- 

 sion with Albert E. Wood concerning these specimens, Wood pointed out that the geological for- 

 mation along Cowhouse Creek is entirely Cretaceous, with no Tertiary strata within many miles of 

 the area. 



These specimens furnish the only basis for including Texas in the known range of any members 

 of this family, and I believe that these skulls were brought into the State from some point farther 

 north or northwest, such as Colorado or Nebraska, and therefore are not indigenous to Texas. 



Merycoidodon culbertsonii periculorum (Cope) 1884 

 PL III, figs. 1-2 



Original Reference: Synopsis of the species of Oreodontidx. Proc. Amer. Philos. Soc, XXI, pp. 511, 

 512, 513 (Oreodon culbertsonl fericuloruni) . 



Type Locality: Horsetail Creek, Logan County, Colorado. Referred material collected in Wyoming 

 and Nebraska. 



Geologic Horizon: Middle Oligocene (lower Brule). 



Type: Holotype, Cat. No. 6397 A.M.N.H., well-preserved skull and jaws, united by matrix but with 

 sutures plainly visible, of a young adult. 



Specific Characters: The skull differs but slightly from that of M. culbertsonii, except in its 

 smaller size. The maximum expansion of the zygomata is just in advance of the glenoid surface. 



