X FROJI MIOHIPPUS TO PROTOHIPPUS 249 



size than Anddtherium, 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 -limbs has become greatly enlarged as compared with the 

 corresponding digit of earlier forms. 



It is held, however, that Amliitherium 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 Besmatipijvs of Professor Scott^ fills in the gap 

 between Miohippus and Protoliippus. 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 FroioMppus. This genus 

 of Horses, of which there is at present but one species, D. 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 ProtoMppius." 



To recapitulate, the following is the probable series of equines 

 in America — IfesoMppus, Miohippus, Besmatippus, Protoliipjms. 



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. American Phil. Soc. xviii. 1896, p. 55. 



