The genus _Desmatippus_ of Professor Scott[162] fills in the gap between _Miohippus_ 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, _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 _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 {250} Ungulate _Phenacodus_ (see p. 202). In the next stage (an embryo of 25 mm.) the humerus has slightly decreased in proportionate length, and has come to be more like that of _Hipparion_. In both of these embryos it should be noted that the ulna is complete and separate from the radius. In the second of the two it has more distinctly acquired the form which it will possess in the adult. The second metacarpal--one of the splint bones of the adult--is tipped with a small nodule of cartilage, which is clearly the representative of one or more of the phalanges belonging to that digit. FAM. 2. TAPIRIDAE.--The Tapirs may be distinguished from the Horse and from the Rhinoceros tribe by a few characters, which are as follows:-- The dentition is generally the full one of forty-four teeth. The premolars in the more ancient forms are unlike the molars, but like them in more recent forms. The molars of the upper jaw have two crests parallel and united by an outer crest. The fore-feet have four, the hind-feet three toes.