544 University of California Publications in Geology [Vol.11 



The presence of a small P 3 in Ischyrosmilus is not a distinguishing 

 character, as this tooth may be present and fully as large in Smilodon 

 calif or nicus. The mandibles of the two genera are distinguished by 

 the greater elongation of the diastema region and the slightly smaller 

 flange in Smilodon. This difference is presumed to represent corre- 

 lation with the large upper canine in Smilodon. 



Although Smilodon is presumed to be derived from Machaerodus, 

 as yet the former has been found only in the New World and the 

 latter in the Old World, and no described form of Machaerodus fur- 



Fig. 155. Ischyrosmilus oshorni, n. sp. Type specimen, mandible with denti- 

 tion, no. 19476, natural size. Kicardo Pliocene, Mohave Desert, California. 



nishes the characters required in the ancestor of Smilodon. As 

 Smilodon is an American genus, and the sabre-tooth is characteristic 

 of America rather than of the Old World, this genus might be pre- 

 sumed to be derived from an American Pliocene form. Of the known 

 American forms Ischyrosmilus most nearly approaches Smilodon, and 

 the similarity of the two genera is closer than is the resemblance of 

 any Old World representative of the genus Machaerodus to Smilodon. 

 This does not mean that Smilodon is derived immediately from 

 Ischyrosmilus. The gap between the two is wide, and it is not im- 

 probable that Ischyrosmihis represents only one specialized branch 

 of a large division leading toward Smilodon. 



