34 



AMERICAN MESOZOIC MAMMALIA 



It may be emphasized that these formulae have been checked wherever necessary 

 by actual dissection of the jaw^s, to make certain that no successional or posterior un- 

 erupted teeth remained uncounted, and that the separation of the genera does not rest 

 solely on the dental formulae. 



Fig. 10. Aj. Posterior portion of right lower jaw of Dasyurus 

 maculatus (Y.P.M.O. No. 253) from the outside. 



Aj. Section of same along line a-b. A, Inflected ridge running 

 forward from the angle. B, Flange bordering the masseteric 

 fossa inferiorly. 



A3. The same jaw from above. 



Bi. Posterior portion of the right lower jaw of Trioracodon 

 bisulcus (restored), from the outside. The oblique dotted line 

 indicates the course of the pterygoid ridge on the inner side of 

 the mandible. 



Eg. Section of same along line a-b. A, Pterygoid ridge, some- 

 times considered homologous with the angle of Dasyurus. B, 

 Flange below the masseteric fossa. (After Simpson, 1925B.) 



Trioracodon bisulcus (Marsh 1880) 



1880. Triconodon bisulcus. Marsh, Amer. Jour. Sci. (3) XX, 237. 

 1928. Trioracodon bisulcus, Simpson, Cat. Mes. Mam. Brit. Mus., 76. 



Type. — Y.P.M. No. 11851. Part of right lower jaw with broken P4 and M1-3. 



Referred Specimens. — Various jaws, both upper and lower. See Simpson 1925B. 



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



Diagnosis. — M3 markedly shorter than M2. Length Mo about 3.1 mm. Molar 

 cusps relatively slender, with straight anterior and posterior contours. Depth of jaw 

 below anterior end of M3, 40-4. 4 mm. 



Priacodon Marsh 1887 

 1887. Priacodon, Marsh, Amer. Jour. Sci. (3) XXXIII, 341. 



Definition. — Dental formula \\ CJ P3 M4. Anterior and posterior molar cusps 

 a little lower relative to the midcusp than in Triconodon. Diastemata following the 

 canines. M* unreduced, tricuspid. M* bicuspid. 



Type. — Priacodon jerox (Marsh). 



Distribution. — Morrison formation, Wyoming. 



