THE MAMMALIA OF THE DEEP KIVER BEDS. 91 



all species of the genus, I cannot at present determine. Another difference from the 

 astragalus of P 'rotohippus is found in the more abrupt truncation of the proximal end 

 of the outer condyle, which thus exposes upon the calcaneum a larger surface for the 

 fibula. As in the corresponding facet of the calcaneum, the sustentacular surface is 

 broader. The navicular surface descends less upon the external side and is separated 

 by a less pronounced angle from the cuboidal facet, which is less extended. The 

 sulcus which invades the navicular surface in both genera is much less conspicuous 

 than in Equus. 



The proximal surface of the cuboid is very similar in the two specimens, except 

 that in Protohippus the posterior extension of the calcaneal facet is somewhat longer 

 and more recurved towards the tibial side. The distal end, on the other hand, is 

 quite different in the two. In Protohippus the facet for metatarsal iv is distinctly 

 larger, and that for mt. iii less oblique and more distal in position, than in Desma- 

 tippus; in the latter the surface for mt. iii is rather lateral than distal, which is an 

 ancient character. 



The navicular is quite different from that of Protohipjnis ; it has a notably 

 greater vertical height and its antero-external angle is much more extended across 

 the face of the cuboid, though far less so than in the horse. Both specimens differ 

 from the latter in the broader posterior portion of the navicular, the relatively greater 

 fore-and-aft diameter of the astragalar surface and in the much less conspicuous 

 development of the beak-like plantar extension. The sulcus on the astragalar facet 

 is also much less marked. 



The enlarged ectocuneiform is very much alike in the two Loup Fork genera. 

 Both differ from Equus in the more rounded and less extended posterior beak ; the 

 articular surface of mt. iii on this extension is continuous with the anterior one, being 

 interrupted by a sulcus on the external side, not, as in Equus, isolated completely. 



The coalesced meso- and entocuneiforms are much larger in Desmaiippus and 

 extend across to the cuboid, with which the internal element is in contact, while in 

 Protohippus they are widely separated and the entocuneiform is much more reduced 

 than in the horse. Both specimens display a minute facet for mt. iii on the middle 

 cuneiform, but the surface for mt. iv is almost confined to this bone and extends but 

 slightly to the internal one, while in the horse it does so largely. 



In the metatarsals the differences are slight, so far as the materials permit a 

 comparison. The cuboidal facet on mt. iii is more oblique and the head of mt. ii is 

 less reduced in Desmatippus. Compared with Equus, more important divergences 

 may be noted ; the cuboidal facet of mt. iii is, in the older genus, much smaller and 

 less proximal in position, and that for the mesocuneiform is also smaller; the posterior 



