8o AMERICAN MESOZOIC MAMMALIA 



lobe does not bear a cusp, but there is a small cuspule on its posterior edge. Between 

 this and the internal cusp, on the posterior rim of the basin, is another very small cusp. 

 The anteroexternal cusp projects sharply and is hook-like, as shown in the figures. The 

 last molar is smaller and somewhat more oblique than the other two. The interdental 

 embrasures are exceedingly narrow. 



Measurements 



Widths of last three molars : i.o, i.o, and 0.9 mm. respectively 

 Length of last three molars, together : 2.2 mm. 



Miccylotyrans Simpson 1927 

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



Definition. — Pantothere upper jaws with seven molars, the posterior ones, at 

 least, markedly broader than long. Crowns similar to those of Euthlastus, but external 

 border straight save for an abrupt median notch on each side of which is an external 

 marginal cusp. A slight rounded transverse median ridge in basin. 



Type. — Miccylotyrans mimmtis Simpson. 



Distribution. — Morrison formation, Wyoming. 



Miccylotyrans minimus Simpson 1927 

 1927. M. minimus, Simpson, Amer. Jour. Sci. (5) XIH, 414. 



Type. — U.S.N.M. No. 2754. Left upper jaw with C, M*", and alveoli. 

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

 Diagnosis. — Sole species of the genus. Lengths of M* to M' about 0.7, 0.7, and 

 0.6 respectively. 



Dentition 



The canine is a relatively large, simply pointed, recurved tooth implanted by two 

 stout divergent roots. There appears to have been a short diastema and then probably 

 four premolars. Back of this the jaw was broken and turned at an angle before or dur- 

 ing burial, so that the cheek tooth formula cannot be certainly determined. There are 

 indications, however, of three molars anterior to those preserved, which, with the three 

 present and one posterior to them, would make seven, probably the correct number. 

 The probable formula is thus T C^ P* M^ 



The first preserved molar, M^ by this count, is worn and broken but seems to have 

 differed little from M*. The latter is a short broad tooth, not unlike a molar of Euthlas- 

 tus in general aspect but quite different in detail. It is somewhat worn, but the facets 

 of wear are easily delimited. They may mark the destruction of other characteristic 

 features, but they do not mislead with regard to what is original and what secondary 

 on the tooth as preserved. The internal cusp is high, slender, and crescentic, its exter- 

 nal face now worn. External to this the crown was shallowly basined, with raised ante- 

 rior and posterior rims as in Euthlastus, but the posterior rim is now truncated hori- 



