82 



AMERICAN MESOZOIC MAMMALIA 



Malthacolestes osborni Simpson 1927 

 1927. M. osborni, Simpson, Amer. Jour. Set. (5) XIII, 414. 



Type.— Y. P.M. No. 13751. Part of right maxilla with M'l 

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

 Diagnosis. — Sole species of the genus. Lengths of first two molars each about 

 1.3 mm. 



Dentition 



M^ is very narrow, being only slightly over half as wide as the following tooth 

 although about as long. The structure shows some analogy with that of Melanodon, 

 but the differences are so marked that this may be misleading. There is a main internal 

 cusp as before, but the anterior crest has apparently been lost, or at least is not present, 

 and the same is consequently true of the basin or depression which is seen between this 

 crest and the median transverse crest in Melanodon. The posterior division of the 



crown, between the median transverse crest and 

 the posterior rim, is, on the contrary, well devel- 

 oped and elongated. The centroexternal and inter- 

 nal cusps are approximated in this very narrow 

 tooth so that their bases are confluent. The former 

 is considerably the lower, is compressed trans- 

 versely, and sends a sharp ridge to the posteroex- 

 ternal cusp. The latter, the extra cuspule postero- 

 internal to it, and the posterior marginal cusp are 

 all present and distinct. The anteroexternal cusp 

 was apparently well developed, but it is broken. 

 M' partakes of the peculiarities of M^ but is somewhat closer to Melanodon. There 

 is a very faint anterior crest on the internal cusp, but it does not reach the centroexter- 

 nal cusp and does not inclose a true basin between itself and the median transverse 

 ridge. The internal and centroexternal cusps are farther apart and are connected by a 

 sharp ridge. There is no anterior ridge or angulation on the centroexternal cusp. The 

 posterior half of the crown is much like that of M* save for its greater transverse width. 



YPM- 137SI 



Fig. 34. Malthacolestes osborni. Right 

 M^"2, crown view, type. 22 times natu- 

 ral size. 



Measurements 



Width Ml 

 Length M^ 

 Width M^ 

 Length M* 



0.7 



ca. 1.3 



i.i 



1-3 



P^Z/co/fjij Simpson 1927 

 1927. Pelicofsis, Simpson, Amer. Jour. Set. (5) XIII, 414. 



Definition. — Pantothere upper jaws. Antepenultimate molar longer than broad, 

 penultimate about as broad as long, and last molar broader than long. Crowns basined, 



