MERYCOIDODON 57 



and extend about 6 mm. forward of the lacrimal bones. The orbits are comparatively small. The 

 temporal ridges unite just behind the postorbital constriction and immediately posterior to the point 

 where the frontoparietal suture crosses the sagittal suture. The low sagittal crest is about a third of 

 the total skull length, while the supraoccipital crest is less produced than in the other species of the 

 genus, and the small wings are wide apart. The brain case is well rounded, with a lateral median 

 convexity. The external auditory meatus is small and directed somewhat upward. The U-shaped 

 palatonarial border is posterior to a line across the posterior of the last molars, and the palate is 

 narrow and slightly vaulted. The basicranial axis is steep. The glenoid articular surface is convex, 

 and the stout postglenoid process has an oblique external border. The paroccipital process is long, 

 triangular in section, and rather deeply excavated on the anteroexternal face, and it extends down- 

 ward and slightly outward. The bulla; are proportionally the largest of any species in this genus. 

 The index ranges from 0.54 to 0.60, young individuals having the shorter and wider skulls. 



Mandible: The symphysis is sometimes fused and has a slope of 43°. There is a very small 

 mental tubercle. The horizontal ramus has a nearly straight inferior border, with the angle faintly 

 marked. There is a large masseteric fossa. The small coronoid process is slender, the small sig- 

 moid notch is shallow and open, and the condyle is small and well inset. The posterior border of 

 the ascending ramus is rugose and thickened. 



Foramina: The infraorbitals lie above P 3 . The supraorbital foramina are usually 10 mm. 

 apart, with well-marked channels running forward and gradually dying out as they bend laterally 

 toward the infraorbital foramina, although in some specimens they reach the latter. The large 

 foramen ovale is directly anterior to the bulla, and the foramen rotundum is moderately large. The 

 posterior palatine foramina are opposite the interval between P 4 and M 1 . 



Dentition: The tooth patterns are like those of M. culbertsonii. The superior molar-premolar 

 index averages about 0.80 and that of the inferior series 0.77. The anterior premolars, especially 

 those of the lower jaw, may be somewhat crowded or even overlap. 



Skeleton: The bones are in general like those of the genotype, except for their smaller size. 

 The first mounted skeleton of this species was placed on exhibition in 1906 by Gilmore at the United 

 States National Museum. It is 690 mm. in length and stands 320 mm. at the shoulder. The Yale 

 specimen is 784 mm. long, with an extreme height of 348 mm., as now mounted. 



Discussion: When Cope in 1884 described his subspecies, M. g. color adoensis, there was 

 apparently a considerable difference in size between his Colorado form and the majority of Leidy's 

 South Dakota and Nebraska specimens. The Cope form was about one-seventh larger. In 1869 

 Leidy figured a specimen, now Cat. No. 136 U.S.N.M., a plesiotype of M. gracilis, which is exceed- 

 ingly similar to Cope's type. However, since then there have been found considerable numbers of 

 skulls of this species which show a wide range of intergradations from the Leidy to the Cope species. 

 There are sexual, individual, and local variations, but they seem to me to be insufficient for division 

 into subspecies. 



Cope (1888, p. 1094) wrote as follows: 



The number of individuals of Oreodontidae which must have existed during the Miocene period in North 

 America is so great as to astonish the paleontologist. During the White River epoch droves of Oreodon culbert- 

 soni inhabited the swamps, and the small O. minor was abundant. Several forms, perhaps species, coexisted with 

 these two. 



This is the only reference to O. minor, and I believe it was a lapsus -pennce for O. gracilis. 

 Diligent search has failed to discover any specimen in Cope's collection which might have been 

 marked O. minor, and there is no reference to it in his notes. Hence we may call it a nomen 

 nudum, probably referring to M. gracilis. 



On a basis of thirty-four individuals in the Marsh Collection it was found that 29 per cent 

 died while in the stage of milk dentition, 50 per cent in the prime of life, 1 1.7 per cent just beyond 

 middle age, and 8.8 per cent after having attained old age. 



