MORRISON MAMMALS: SYMMETRODONTA 



39 



^^^^^^ MN^rOi 



Tiuodon and Menacodon 

 were referred by Marsh to dif- 

 ferent families, by Osborn to 

 distinct subfamilies, but they 

 prove to be synonymous. Marsh 

 did not directly compare them, 

 and reexamination shows con- 

 clusively that the apparent dis- 

 tinctions are due solely to 

 slight differences in preserva- 

 tion. Only two species can now be recognized, and these are doubtfully distinct. 



TINODON 



Fig. 15. Tinodon. Diagrammatic internal view of right 

 lower jaw. Three times natural size. 



Tinodon bellus Marsh 1879 

 1879. T. bellus, Marsh, Amer. Jour. Set. (3) XVIII, 216. 



Type. — Y.P.M. No. 1 1843. Right lower jaw in matrix, inner aspect, with M1.4. 



Referred Specimen. — Y.P.M. No. 13644. Left lower jaw in matrix, inner as- 

 pect, with P1.3 and M1-4. 



Horizon and Locality. — y^WKh'^* 



Morrison formation, Quarry 9, 

 Como Bluflf, Wyoming. 



Diagnosis. — Mi with round- 

 ed relief, faint cingulum, no 

 trenchant edge on main cusp. 

 M2-3 with relatively broad cin- 

 gula. 



The above diagnosis sepa- 

 rates the species from T. lefidus, 

 but it is very probable that these 

 differences are due solely to wear 

 or to state of preservation. 



^^2=^2:2C^^iM^. 



Fig. 16. Tinodon bellus Marsh. A, outline restoration of 

 mandible, internal aspect, X3. DF, dental foramen; IG, 

 internal groove; PR, pterygoid ridge; SY, symphysis. B, 

 posterior view of condyle, much enlarged. Ex : external 

 side. In: internal side. (After Simpson, 1925c.) 



Tinodon lepidusM.zr%h. 1879 



1879. T. lepdus, Marsh, Amer. Jour. Set. (3) XVIII, 398. 



1887. Menacodon rarus, Marsh, Amer. Jour. Set. (3) XXXIII, 340. 



Type. — Y.P.M. No. 11845. Part of left lower jaw in matrix, inner aspect, with 

 broken Mi and M3. 



Type of Menacodon rarus. — U.S.N.M. No. 2131. Part of left lower jaw with 



C, Pi-3andMi-3. 



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



Diagnosis. — Mi with sharp relief, sharp cingulum, trenchant edges on main cusp. 

 M2-3 with somewhat narrower cingula. 



