No. 3.] OSTEOLOGY OF MESOH1PPUS AND LEPTOMERYX. 333 



with that of the ectocuneiform. The distal surface has two 

 facets, for the ectocuneiform and the coalesced meso- and ento- 

 cuneiforms respectively. The former is broad, and sends out 

 a narrow posterior prolongation. The latter is much smaller 

 and regularly convex from before backwards, and does not show 

 any definite separation into facets for the two elements of the 

 compound cuneiform. 



The navicular of Anchitherium is broader and lower, and the 

 antero-external angle is very much produced, encircling the 

 cuboid in a curious way (Kowalevsky, PI. III., Fig. 17). The pos- 

 terior prolongation of the facet for the ectocuneiform is curved 

 sharply toward the fibular side, and the facet for the first and 

 second cuneiforms are in contact, though their limits are defined 

 by a ridge. There is no fossa in the astragalar surface. 



In Equus the navicular is still broader and lower ; it has quite 

 a large anterior facet for the cuboid, and the posterior facet has 

 become more prominent. The elevations of the posterior proxi- 

 mal margins and the depression between them are hardly more 

 marked than in Mesohippas, but have become decidedly more 

 massive and rugose. The fossa in the astragalar surface is 

 deep and conspicuous and continuous over the fibular margin. 

 The facet for the coalesced cuneiforms, especially for the 

 entocuneiform portion, has become decidedly larger ; but the 

 principal increase in the breadth of the navicular is, as Kowalev- 

 sky has suggested, in that portion of it which supports the 

 ectocuneiform. 



In Mesohippus, the ento- and mesocuneiforms have coalesced. 

 The disposition of the equine tarsus to become reduced in 

 breadth, but increased in depth, and to close behind so as to form 

 a circle, has been pointed out by Riitimeyer (No. 46, p. 17), and 

 is already very apparent in the White River genus. The ento- 

 cuneiform is high and compressed, and sharply everted so as 

 to be in contact with the cuboid ; it greatly exceeds the meso- 

 cuneiform in vertical height, rising much above it on the navic- 

 ular and descending much below it distally. The navicular 

 facets of the two elements are confluent, but the distal facets 

 are very distinctly separated ; of the latter there are two only, 

 one on the distal surface of the mesocuneiform for the head of 

 the second metatarsal, and one on the anterior face of the ento- 

 cuneiform for the hinder edge of the same metatarsal. There 



