338 M. R. Thorpe — A Netv Merycoidodon, 



Merycoidodon macrorJiifms (Douglass) 1902. 



ilf. macrorhinus is apparently not represented in this 

 collection. It is a Middle Oligocene form from the nodular 

 layer in Montana, collected near the Missouri River, 

 southeast of Helena, and the holotype is Carnegie Museum 

 Cat. No. 767. In nearly all respects it is close to M. cul- 

 hertsonii, except that the face is one third higher above 

 P^ and the bone throughout is heavier. It may represent 

 a male, showing an example of a considerable degree of 

 dimorphism, but as I have not seen any comparable forms 

 in the White River deposits, nor any examples of marked 

 sex dimorphism in this group, it seems that the species is 

 valid. The length of the superior molar series is 45 mm. 

 and the length of the superior premolar series is the same. 



Merycoidodon gracilis (Leidy) 1851. 



This Middle Oligocene form has more than sixty 

 representatives in the Marsh Collection from Colorado, 

 Nebraska, Wyoming, and South Dakota. There seems 

 to be such close resemblance between this species and M. 

 color ado ensis (Cope) that they should be considered 

 synonymous. The main distinctions of the latter were 

 stated to be a little larger size, about 5 mm. longer in total 

 length, and a different geographic locality, Colorado, to 

 which Cope considered the species confined. 



Another species is mentioned by Cope, and defined as 

 follows (1888, p. 1094) : ^'During the White River epoch 

 droves of Oreodon culbertsonii inhabited the swamps, 

 and the small 0. minor was abundant. Several forms, 

 perhaps species, coexisted with these two.'' Since this 

 form is defined only to the extent of being smaller than 

 M. culbertsonii, it is my opinion that 0, minor should .be 

 considered a nomen nudum. 



The amount of variation within this species seems not 

 to be sufficient for subdivision into varieties. The young 

 forms with deciduous dentition are usually more dolicho- 

 cephalic and the posterior nasal bones are somewhat more 

 acute than in the adult. There are likewise variations in 

 the adult forms, but after a careful study of the specimens 

 in this Museum, I am led to the conclusion that these dif- 

 ferences are not greater than we should expect to find. 



