576 LAND MAMMALS IN THE WESTERN HEMISPHERE 



Although the actual line of descent was not through these 

 genera, the series, \Sinopa — ]Tritemnodon — \Pterodon — 

 ^Hijcenodon, extending from the lower Eocene into the Oligo- 

 cene, displays perfectly the successive steps in the transforma- 

 tion of the teeth. The skull underwent a corresponding series 

 of changes, ending in long-faced, long-jawed, wolf -like forms, 

 with larger brain-case than in any other fcreodonts. The 

 elongated form of body was retained, but the tail was reduced 

 to moderate proportions. The limbs and feet did not change 

 greatly, except in size and in the greater bluntness of the claws. 

 The fMiacidse, if not actually referable to the Fissipedia, 

 at least anticipated them in the mode of carnassial develop- 

 ment. The upper molars changed very little from the primi- 

 tive tritubercular plan, but the fourth upper premolar was en- 

 larged and acquired a trenchant ridge behind the original single 

 outer cusp. The lower molars were at first all alike, except in 

 size, the first being the largest ; they had the primitive pattern 

 common to the earlier members of nearly all the fcreodont 

 families, of an elevated anterior triangle of three subequal 

 cusps and low, basin-like heel. The first molar grew larger 

 in the successive genera and, by the enlargement of the two 

 external cusps of the primitive triangle and reduction of the 

 inner one, gradually became an efficient sectorial, the fourth 

 upper premolar keeping pace with it. In proportion as the 

 first lower molar was elaborated, the second and third were 

 reduced in size and the anterior triangle was lowered to the 

 level of the heel, these teeth thus becoming tubercular. All 

 the jMiacidse were small animals, none attaining the stature 

 of a fox, though some had heads as large. From this family, 

 as was pointed out above, probably arose all of the Fissipedia, 

 the history of which it is needless to repeat. 



