MORRISON MAMMALS: PANTOTHERIA 



8i 



zontally and the anterior rim is obliterated, its place taken by a strongly oblique facet 

 (from wear against the pr<'-me'' crest of the lower molar). The central part of the basin 

 is occupied by a small but distinct rounded ridge running straight externally from the 

 base of the internal cusp but almost disappearing before reaching the external border. 

 This border would be nearly straight were not its contour broken in the center by a 

 small, abrupt notch. On each side 

 of this notch is a marginal cusp, the 

 posterior one in line with the me- 

 dian transverse ridge, which does 

 not quite reach it. There is a hook- 

 like anteroexternal cusp like that of 

 Euthlastus. Along the posterior bor- 

 der there is one marginal cusp some- 

 what internal to the outer border 

 and, although it is now truncated by 

 wear, there is clear evidence of an- 

 other, probably larger, posterior 

 marginal cusp between this and the internal cusp. 



M* is a smaller tooth and its external border is more oblique. It was later in erup- 

 tion, being at a slightly lower level and less worn. The external border is not definitely 

 notched, but is somewhat concave in the center. The anterior of the two centroexternal 

 cusps is prominent, but the posterior one is small. The anteroexternal cusp is relatively 

 broader and less hooked. The raised posterior rim bears one cusp at about its midpoint, 

 but the end of this ridge externally is elevated and is no doubt homologous with the 

 more external posterior cusp of M^. 



Judging from its apparently confluent alveoli, M7 was very small. 



Measurements 

 Miccylotyrans is one of the smallest known mammals. 



Fig. 33. Miccylotyrans minimus. Left M"'". Type. A, 

 crown view. B, external view. 25 times natural size. 



Malthacolestes Simpson 1927 

 1927. Malthacolestes, Simpson, Amer. Jour. Set. (5) XIH, 414. 



Definition. — Pantothere upper jaws. First molar much and second molar a little 

 longer than broad. On these teeth, median transverse crest present, but part of crown 

 anterior to it much reduced and part posterior to it large and basin-like. Interspace 

 between M' and M'' long. 



Type. — Malthacolestes osborni Simpson. 



Distribution. — Morrison formation, Wyoming. 



