126 AMERICAN MESOZOIC MAMMALIA 



Horizon and Locality. — Lance, Niobrara County, Wyoming. 

 Diagnosis. — Preferably used, sensu lato, for all members of the genus. 



Included Species 



1927. Ectoconodon montanensis, Simpson, Amer. Mus. Nov., No. 267, p. 5. Type: Amer. Mus. 

 No. 14406, isolated left upper molar from Crooked Creek, Dawson County, Montana. 



The larger and more distinctive of Osborn's synt):pes is here made lectotype, since 

 it is apparent that his two specimens do not fall within the ordinary range of a single 

 species. E. montanensis Simpson is distinct and it will be necessary to use the name 

 when the Hell Creek mammalian fauna is better known, but more mature considera- 

 tion persuades the writer that the attempted separation of this species is not useful at 

 the present time. There are thus at least three species in the genus, all included under 

 E. ■peter soni sens. lat. 



The trigon is like that of Didelfkodon vorax save that the primary cusps are a 

 little more pointed and the disparity between the lower pa and higher me is somewhat 

 greater. The contour of the external shelf is almost identical, but the styles are rather 

 different. A is even smaller. D, at best merely indicated in Didelfhodon, is here a 

 prominent cusp nearly as large as B and larger than the pa. The internal slope of D is 

 long, and on it there is a small lesser tip just internal to the main one. The crest-like E 

 is strongly marked. In addition there is a small cusp on the shelf between B and D and 

 between the pa and me. It is very small and is connected by ridges to the latter two 

 cusps, being considerably internal to the margin of the tooth. 



Close comparison with Didelfhodon, Thlaeodon, and Pediomys, genera in which 

 the orientation of the upper molars is certainly known, shows that Osborn's orientation 

 of his isolated Ectoconodon teeth was in error, — both of his specimens are right upper 

 molars, not left. 



Alphadon Simpson 1927 

 1927. Alfkadon, Simpson, Amer. Jour. Sci. (5) XIV, 125. 



Definition. — Didelphodontine upper molars. Stylar cusps seen in Didelfhodon 

 less prominent than in that genus, but with a very strong median external cusp. Spe- 

 cies of relatively small size. 



This genus includes the smallest of the known didelphodontines. It seems to be 

 rather sharply delimited and it differs more from the preceding three genera than do 

 the latter among themselves. 



Alphadon marshi Simpson 1927 

 1927. A. marshi, Simpson, Amer. Jour. Sci. (5) XIV, 125. 



Type. — Y.P.M. No. 13659. Isolated upper molar. 



Horizon and Locality. — Lance, Niobrara County, Wyoming. 



Diagnosis. — Sole species now referred to the genus. 



