44 THE INTERNATIONAL MONTHLY. 
backward, and the growing postorbital process more nearly 
encloses the eye with a bony rim. The brain case is much enlarged, 
and the brain itself is not only larger, but is much more abun- 
dantly convoluted than in the Eocene genera of the series. The 
neck has grown longer, and its vertebrae have already assumed 
many of the peculiarities seen in the modern horses. The limbs, 
especially the hinder ones, are relatively much longer than in the 
preceding genera of the line, and the feet have lengthened pro- 
portionately even more than the limbs as a whole, and are thus in 
very marked contrast to those of the more ancient genera. In 
the forearm the radius has been much enlarged, and carries most 
of the weight of the body, while the ulna is much reduced and 
very slender; the two forearm bones have already begun to co- 
ossify. Similarly, in the leg the tibia is enlarged at the expense 
of the fibula, the shaft of which has become a mere thread of 
bone, while the two ends have coalesced with the tibia. The 
fore foot has lost the fifth toe, which is reduced to a mere nodule ; 
the median digit (no. iii. of the original five) has grown much lar- 
ger, and carries most of the weight, while the lateral digits (nos. 
ii. and iv.), though still complete and still functional, are much 
reduced in thickness and are evidently on the way to disappear. 
The median hoof has now assumed an unmistakable resemblance 
to that of the horse, while the two lateral hoofs have become very 
narrow and slender. Similar changes have taken place in the 
hind foot. In short, Mesohippus, of the White River beds, as 
compared with the genera of the Wasatch and Bridger, has made 
very notable progress, in all parts of its structure, in the direction 
of the modern horses. 
Through the White River and John Day beds the advance of 
evolution continues in the same steady fashion, a gradual increase 
of size being the most conspicuous feature of the change within 
the limits of these formations. In the Deep River beds the 
change has become so marked that a new generic term becomes 
necessary. This genus (Desmatippus) still closely resembles its 
ancestral type in most characters of skull and skeleton, but it dis- 
plays one very significant change ; namely, in the constitution of 
