No. 3.] OSTEOLOGY OF MESOHIPPUS AND LEPTOMERYX. 34 1 



while in the recent type the trochlea is massive, very promi- 

 nent, and obliquely directed, the inner margin being much 

 higher than the outer. The tibia is long, but very slender, 

 except in the upper portion, which is deepened antero-posteriorly 

 by the large cnemial crest. The fibula may have been complete; 

 at all events, the distal end is quite large, and in several of the 

 specimens a considerable length of the very slender shaft is 

 attached to it. In some individuals the distal ends of the tibia 

 and fibula are co-ossified, in others they are separate, and this 

 does not appear to be altogether a matter of age, but rather of 

 individual variability. The tarsus is relatively higher and 

 narrower, and of less antero-posterior extent than in the recent 

 forms ; the calcaneum longer and less massive. The metatarsus 

 is of nearly the same length proportionately as in the horse, but 

 the third digit is very much less enlarged, while the second and 

 fourth, as in the manus, are functional and of nearly the same 

 length as the third. The enlargement of the median meta- 

 tarsal has proceeded somewhat farther than in the case of the 

 corresponding metacarpal. 



As the restoration is drawn from several individual speci- 

 mens, reduced to a common average standard, it is impossible 

 to give the relative dimensions of the parts with any great 

 accuracy, but, on looking over a large number of specimens, it 

 is clear that the hind limbs are decidedly longer, in proportion 

 to the fore limbs, than in the recent species. In the figure this 

 is compensated by the degree of flexion given to the posterior 

 extremities, though very possibly the hind quarters were much 

 more elevated, in somewhat the same fashion as in the trasu- 

 lines, though of course in a less marked degree. 



There are thus many points of difference, as regards the 

 proportionate development of the various parts of the skeleton, 

 between Mesohippus and Equus, and these divergences, more 

 especially the smaller and differently shaped head and the very 

 slender tridactyl feet, give to the older type quite another 

 physiognomy than that of the recent representatives of the 

 group, even without taking into consideration its very much 

 smaller size. Nevertheless, no one can examine the skeleton of 

 the Miocene genus without being struck by its essentially equine 

 nature ; in the teeth alone is the fundamental similarity of plan 

 not apparent at the first glance, though even here a careful 



