x FROM A//OATPPUS TO PROTOHIPPUS 249 
size than <Anchitherium, to be considered immediately. The 
odontoid process of the axis is just beginning to assume the 
characteristic spout-like shape of that of the existing Horse and 
many modern Ungulates. The median digit of both fore- and 
hind-lmbs has become greatly enlarged as compared with the 
corresponding digit of earlier forms. 
It is held, however, that Anchitheriwm is not on the direct 
line of descent either in America or in Europe, in both of which 
it occurs. Its teeth are in some respects less Horse-like than in 
some of the more ancient genera, to which the converse would be 
expected on the descent theory. Its hoofs are much elongated 
and flattened, a mark of specialisation and not appropriate to 
a creature holding an intermediate position in the equine series. 
Both the American (A. equinum) and the European species (A. 
aureliense) are of very large size, larger than its successors, and 
such “ alternations in bulk are unlikely.” 
The genus Desmatippus of Professor Scott’ fills in the gap 
between JMohippus and Protohippus. The molars and premolars 
are brachyodont, but there is a thin deposit of cement in the 
tooth valleys, leading towards the more complete filling of these 
valleys with cement, which is found in Protohippus. This genus: 
of Horses, of which there is at present but one species, J). crenidens, 
was three-toed, and “the lateral digits, so far as can be judged by 
fragmentary remains, were still fairly developed, and though 
much more reduced than in Miohippus, appear to be somewhat 
less so than in Protohippus.” 
To recapitulate, the following is the probable series of equines 
in America— Mesohippus, Miohippus, Desmatippus, Protohippus. 
The development of the limbs of the Horse shows a most 
interesting series of stages, which correspond in part to the 
ancestral forms which palaeontology seems to prove to be the 
line of the descent of our existing Equidae. This matter. has 
recently been elucidated by Professor Ewart, who details the 
following facts and comparisons :— 
In the. youngest embryo (about 20 mm. in length) the 
humerus is somewhat curved, and considerably longer than the 
radius and carpus taken together. The first-named bone is 
shorter in the adult, and the proportions of that bone in the 
-young as well as its curvature are suggestive of that ancient 
! Trans. Amcrican Phil. Soe. xviii. 1896, p. 55. 
