ized character and probably later age. The nine species of Menodus may be compared 

 with five of those of Symborodou, as follows : — 







Nasals long ; horns short 



Nasals and horns 

 intermediate. 



Nasals short ; horns long. 





f Nasals transversely an- ") 



if. americanus. 

 if. cohradoensis- 

 if. angustigenis. 

 if. selwynianus. 



if. syccras. 

 if. proutii. 

 if. tichoceras. 



if. plalyceras. 



Menodus - 



Nasals not angulated ■ 



I 



if. dolichoceras. 



Syniborodo 



Q l 



I Nasals not angulated .... 



S. trigonoceras. 

 S. bucco. 



S. altirostris. 



S. acer. 



Apart from the generic characters, the Menodus americanus, Leidy, appears to be an 



enlarged S. trigonoceras, Cope ; and the M. tichoceras, S. and O., an enlarged S. altirostris, 



Cope. Apart from these, the correspondences are not so close. A definite character 



which divides Menodus into two groups is the presence of an internal cingulum of the 



premolars in some of the species. The same character divides the genus Symborodou. 



According to this character, the species may be grouped as follows, so far as they are 



known in this respect : — 



Menodus. Symborodon. 



(" if. proutii 



Without cingulum . 



if. tichoceras 



S. bucco 

 S. altirostris 



(if. 



americanus 



With cingulum -{if. coloradoenm 



I 



(_ M. angustigenis 



S- trigonoceras 

 S. heloceras 



The collections of the Geological Survey include fragments of skeletons of five species 

 of Menodontidae, which I shall refer, some of them provisionally, to the genus Menodus. 

 The M. americanus is the only one known to have possessed horns of triangular section. 

 The section is oval in the M. proutii and M. angustigenis. In M. selwynianus the horns 

 are unknown. 



In the Annual Eeport of the United States Geological Survey of the Territories for 

 1874, p. 480, I gave a general account of the osteology of the genus Symborodou, which 

 applies equally well to the nearly allied genus Menodus. I will now add a few points 

 not referred to in that report, derived from the specimens here described. 



The condyles of the humerus have no trace of trochlear crest. The olecranon is 

 expanded laterally and vertically at the extremity. The head of the radius is only con- 

 vex below. Its carpal extremity is narrowed inwards, and it is bounded below its middle 

 by a fossa. The scaphoid and lunar parts of the surface are not distinguished. 



The third trochanter of the femur is not a process but an angle, projecting but little 

 beyond the external face of the femur above it, but bounding a contraction of the dia- 

 meter below it. The little trochanter is insignificant. The rotular surface is grooved 

 medially, and the lateral ridges are prominent, especially aboA r e and proximally, where 

 they rise abruptly from the shaft, which has at this point, and between them, a fossa. 

 2 



