68 



KANSAS CITY REVIEW OF SCIENCE. 



point observable in some species of this genus, probably represents it, however. 

 The anterior lobe x is strong. The four principal cusps ae, pe, ai and pi are 

 more elevated and conic, giving to the intervening spaces or valleys, greater 

 depth. The cross crests ace and pec assume more distinct proportions and tend 

 to connect the internal with the external cusps. In the inferior molars the four 

 principal lobes are higher and are connected by cross crests. The antero-poster- 

 ior crest z passing from the postero-external to the antero- internal, cusps is much 

 better. developed. The antero-internal cusp is sHghtly bifid at its extremity and 

 is traversed by a vertical groove on its inner side. A well-marked ridge k de- 

 scends from the antero-external cusp ae, and becomes continuous with the cin- 

 gulum below. This ridge is absent in Phenaeodus. 



The feet (Fig. 8 and 9) show a reduction of one digit from the anterior and two 

 from the posterior limbs. This genus is found in the lower Eocene, but a closely 

 allied form is found in the upper Eocene beds. 



Figs. 8 and 9. 



Left fore and hind feet; of Hyracotherium venticolum, Cope. One-half natural size, from Cope, 



The next step in specialization we observe in the teeth of Chalieotheriidce, a 

 family in which the antero- and postero-external cusps of the superior molars are 

 separated by an external vertical ridge (Fig. 10) _y. The inferior molars Fig. 11 



