20 AMERICAN MESOZOIC MAMMALIA 



Ctenacodon scindens Simpson 1928 

 1928. Ct. scindens, Simpson, Cat. Mes. Mam. . . . Brit. Mus., p. 36. 



Type. — Cat. No. 10366, Y.P.M. Part of left lower jaw, free of matrix, with P3-4, 

 Mi-2 and roots of P1.2. 



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

 ming. 



Diagnosis. — Length P1-4 about 4.6 mm. Length ratio P4: P3, 182. Premolar ser- 

 rations as in Ct. serratus. P4 higher relative to length than in other known species. 

 Anterointernal cusp of Mi larger than in Ct. serratus. 



Dentition 



The two premolars preserved are identical in pattern with those of Ctenacodon 

 serratus, but differ in proportions. Both are higher relative to their lengths than in the 

 genotype, and P3 is shorter relative to P4. 



Ml has the anterointernal cusp better developed than in the preceding species, 

 although it is less distinct from the second internal cusp than is the third. The external 

 row is of three rounded and much worn cusps, the anterior the smallest, as before. M2 

 is broader and more rounded in outline than Mi. The inner cusps are separated by 

 deep notches, although here it is the second notch and not the first which is shallower. 

 There are three subequal outer cusps. 



The dental series as a whole is about 12 per cent or more larger than in Ct. serra- 

 tus, but it is somewhat smaller in proportion to the depth of the jaw. 



Mandible 

 The lower jaw was apparently shorter and heavier in proportions than in the type 

 species. The outer ridge alluded to in the description of the latter is here much less 

 marked and is scarcely to be distinguished until it rises to form the anterior border of 

 the coronoid. 



Measurements 

 All measurements from the type: 



