32 



AMERICAN MESOZOIC MAMMALIA 



B 



Fig. 8. Morrison Amphilestinae. 

 External views of penultimate 

 lower molars. A, Aploconodon 

 comoensis. B, Phascolodon gid- 

 leyi. A, ten times natural size. B, 

 eleven times natural size. (After 

 Simpson, 1925B.) 



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

 Diagnosis. — Sole species of the genus. Lengths of last three molars: 1.6, 1.2, and 

 0.7 mm. respectively. 



Afloconodon'$)VCS\^%on 1925 

 1925. Afloconodon, Simpson, Amer. Jour. Sci. (5) X, 336. 



Definition. — (Number of molars unknown.) Accessory cusps very minute, less 

 than half height of main cusp, the latter more acute and slender than in Phascolodon. 



Molar cingulum obsolete in middle and forming no 

 cingulum cusps. Last molar much reduced in size, w^ith 

 relatively higher accessory cusps than on the preceding 

 tooth. 



Type. — A. comoensis Simpson. 

 Distribution. — Morrison formation of Wyo- 

 ming. 



Although very poorly known, this extraordinary 

 genus is very distinctive. It is the only known tricono- 

 dont in which cingulum cusps are missing on the pe- 

 nultimate molar, and on this same tooth the accessory 

 cusps are smaller than in any other known genus of the 

 group. This tooth is suggestive of the premolars of the Stonesfield triconodonts and it 

 is either a remarkable example of the survival of a primitive type or a still more re- 

 markable example of secondary simplification. The presence of the last molar in the 

 only known specimen fortunately removes all doubt as to identification. 



Afloconodon comoensis Simpson 1925 

 1925. A. comoensis, Simpson, Amer. Jour. Sci. (5) X, 336. 



Type.— U.S.N. M. No. 

 2791. Fragment of right 

 lower jaw with last two mo- 

 lars. 



Horizon and Local- 

 ity. — Morrison formation. 

 Quarry 9, Como Bluff, Wyo- 

 ming. 

 Diagnosis. — Sole species of the genus. Lengths of last two molars: i.i and 0.7 

 mm. respectively. 



TRICONODONTINAE Osborn 1907 



Definition. — Premolars asymmetrical, more or less recurved, the last ones 

 higher than the first molars. Molars four or three in number, with anterior and pos- 

 terior cusps nearly or quite equal to the midcusp. 



Fig. 9. Cynodonts and triconodonts. Internal views of lower mo- 

 lars. A, Ictidopsis. B, Microconodon. C, Amphilestes. D, Phasco- 

 lothertum. E, Priacodon. F, Triconodon. Not to scale. 



