286 Wortman — Studies of Eocene Mammalia in the 



them as a subfamily of the Mesonychidse. The differences 

 which distinguish them from the Mesonychinae are not great, 

 and consist in the deep, heavy lower jaws, with powerful sym- 

 physis, together with the much wider, more typically trituber- 

 cular upper molars, as well as the more subequal size and 

 normal arrangement of the cusps of the tn'gon of the lower 

 molars. 



While the Triisodont division became extinct at the close of 

 the Torrejon, in Dissacus we have the beginning of the Mesonyx 

 line, which continued through the whole of the Eocene, being 

 represented in the several stages by a number of species and 

 genera, and towards the close of its career, as we shall pres- 

 ently see, developed a limb structure almost equaling, if not 

 actually surpassing, that of the modern dogs in point of special- 

 ization for a running habit. The connections between these 

 two extremes are close, the interval being completely bridged 

 by the Wasatch Dackycena. In Dissacus, the feet are rela- 

 tively short and stout ; they are pentadactyle and the toes were 

 spreading, a condition which was gradually modified into the 

 four-toed, compressed, elongated feet of the cursorial Mesonyx. 

 I define the known genera of the Mesonychinae as follows : 



I. Digits, 5-5 ; humerus with an entepicondylar 



foramen. 



a. Internal cusps of inferior molars distinct ; 



posterior external cusps of superior 



molars smaller than anterior ; M.J, Pm.f Dissacus. 



b. Internal cusps of inferior molars vestigial; 



external cusps of superior molars equal; 



M.f, Pm.f .... Pachymna. 



c. Internal cusps of inferior molars vestigial ; 



external cusj)s of superior molars equal ; 



M.-f, Pm.f a) Harpagolestes. 



II. Digits 4-4; humerus without entepicondylar 



foramen. 



a. M.f Dromocyon. 



b. M.f _ . . 3fesoi<yx. 



Subfamily Mesonichince. 



Harpagolestes macrocephalus gen. et sp. nov r . 



The remains upon which this genus and species are founded 

 consist of the greater portion of a skull, together w T ith a com- 

 plete humerus of the right side, a distal end of a femur, and a 

 centrum of an axis, all belonging to one individual. With the 

 exception of considerable vertical crushing, the anterior portion 

 of the skull is well preserved, including most of the teeth. 

 The posterior part of both mandibular rami are present, with 



