M. R. Thorpe — A New Merycoidodon. 339 



due to sexual, individual, and local variations of minor 

 importance. 



Using a considerable series of specimens, it was found 

 that the length of the superior dental series ranged 

 from 57 to 60 mm. ; that of the molars, from 33 to 37 mm. ; 

 and of the premolars, from 27 to 30 mm. 



On a basis of thirty-four individuals, the age classes 

 show: 1-3, 10 individuals, or 29 per cent; 4-6, 17, or 50 

 per cent ; 7-8, 4, or 11.7 per cent ; and 8-10, 3, or 8.8 per 

 cent. This series does not represent as great a number 

 of individuals as might be desired, but it shows a varia- 

 tion in the first two classes, while the latter two are more 

 nearly normal. 



Merycoidodon affinis (Leidy) 1869. 



The genus Merycoidodon is characterized mainly by 

 dolichocephalic forms, but M. affinis is mesaticephalic. 

 The muzzle is short and the diameter across the frontals 

 at the, orbits is as great as in M. culbertsonii. The total 

 length is but slightly greater than that of a robust M, 

 gracilis. The length of the superior dental series 

 averages 67.5 mm. ; that of the molars, 38 mm. ; and of the 

 premolars, 32 mm. 



There are some twenty or more skulls and parts of 

 skulls here, together with much skeletal material, collected 

 in Colorado and Nebraska. M. affinis differs from the 

 other species of the genus in that it is Lower Oligocene 

 (Chadron) in age. 



Merycoidodon platycephalus, sp. nov. 



(Figs. 1, 2.) 



Holotype, Cat. No. 12752, Y. P. M. Middle Oligocene (lower Brule), 

 near Scott 's Bluff, Nebr"&.ska. Skull and some skeletal parts. 



Distinctive characters. — Very low, wide cranium; long 

 sagittal crest ; nuchal crests wide apart and having less 

 overhang than in other species of this genus ; posterior 

 part of nasal bones obtuse ; infra-orbital foramen above 

 interval between P^ and P^; lacrymal fossa small and 

 shallow ; orbits look forward and outward only, and their 

 outline is quite different from that of any other species in 

 the genus (see %. 1) ; orbital opening smaller than in M. 

 gracilis ; supra-orbital groove a well defined depression 



