MORRISON MAMMALS: PANTOTHERIA 75 



Melanodon Simpson 1927 

 1927. Melanodon, Simpson, Anter. Jour. Set. (5) XIU, 4i3- 



Definition. — Dryolestid upper jaws with seven molars, each markedly shorter 

 than broad. Centroexternal cusp much stronger than in Herfetatrus, more median, less 

 external. Strong posterior marginal cusp and weak anterior marginal cusp. Strong, 

 sharp median transverse crest. Last premolar without internal cingulum; external 

 cingulum with three cusps — anterior, central, and posterior. 



Type. — Melanodon oweni Simpson. 



Distribution. — Morrison formation, Wyoming. 



As pointed out by Gregory on the basis of drawings by 

 the present writer, this strongly characterized genus seems 

 to be of peculiar interest from the standpoint of molar evo- 

 lution (Gregory 1926, fig. 3, IH, left hand figure). Two Fig. 29. Comparison of up- 

 species are recognized, differing in details of molar struc- ^^\^^^^'l^^ "'ZlTn' 

 ture. It is probable that this is the upper dentition either of (right). 

 Dryolestes or of Laolestes. 



Melanodon oisoeni Simpson 1 927 

 1927. Melanodon oweni, Simpson, Amer. Jour. Set. (5) XIH, 413. 



Type.— Y.P.M. No. 10663. Part of right maxilla with P* and M^■^ 

 Horizon and Locality. — Morrison formation, Quarry 9, Como Bluff, Wyoming. 

 Diagnosis. — Centroexternal cusp subconical. Outer border of tooth emarginate 

 opposite this cusp. (A number of lesser diflferences, see below.) 



Dentition 

 The main cusp of P* is nearly a true cone, not at all compressed, although it is 

 slightly recurved and has a subtrenchant posterior edge. There is a feeble posterointer- 

 nal cingulum, now much obscured, but no true internal cingulum. The external cingu- 

 lum. on the contrary, is strongly developed, sharp, continuous, and horizontal. It 

 widens and rises in three places to form distinct anteroexternal, centroexternal, and 

 posteroexternal cusps. The arrangement and ground plan of the molars are the same as 

 in Herfetairus, but the detailed structure has numerous well defined and important 



differences. 



The raised anterior rim of the basin runs to the centroexternal cusp and up its 

 anterointernal slope to the apex. Halfway between this and the internal cusp it bears a 

 small cuspule, hardly distinguishable on M^ but progressively stronger until on M' it is 

 about half as large as the posterior intermediate cusp. It is not crescentic, but semi- 



