. The Cambridge natural history. Zoology. NUMBER OF TOES 43. the )s must have preceded the fore-liinbs in their thorough adaptation to the cursorial mode of progression. In the Mammalia the ankle-joint is always wliat is termed cruro-tarsal, between the ends of the limb-bones and the proximal row of tarsals ; not in the middle of tlie tarsus as in some Sauropsida, (reptiles and birds). The bones of the ankle are much like those of the hand ; but there are never more than two bones in the proxi- mal row, whicli are the astragalus and the calcaneum. The former is perhaps to be loo
. The Cambridge natural history. Zoology. NUMBER OF TOES 43. the )s must have preceded the fore-liinbs in their thorough adaptation to the cursorial mode of progression. In the Mammalia the ankle-joint is always wliat is termed cruro-tarsal, between the ends of the limb-bones and the proximal row of tarsals ; not in the middle of tlie tarsus as in some Sauropsida, (reptiles and birds). The bones of the ankle are much like those of the hand ; but there are never more than two bones in the proxi- mal row, whicli are the astragalus and the calcaneum. The former is perhaps to be looked upon as the equivalent of the cuneiform and lunar together. But the views as to the homologies of the tarsal Vtones differ widely. Below these is the na^'icular, regarded as a centrale. The distal row of tlie tarsus has four bones, three cuneiforms and a cul:)oid. Eeduction is effected by the soldering together of two cuneiforms as in the Horse, by the fusion of the navicular and cuboid as in the Deer. No mammal has more than five toes, and the number tends to become rediiced in cursorial animals (Eodents, Ungulates, Kangaroos). Teeth.âThe teeth of the Mammalia ^ differ from those of other vertebrated animals in a number of important points. These, however, entirely concern the form of the adult teeth, their position in the mouth, and the succession of the series of teeth. Developmentally and histologically there are no funda- mental divergences from the teeth of vertebrates lower in the scale. In mammals, as for example in the Dog, the teeth consist of three kinds of tissueâthe enamel, the dentine, and the cement. The enamel is derived from the epidermis of the mouth cavity, and the two remaining constituents from the underlying dermis. The teeth originate quite independently of the jaws, with whicli they are later so intimately connected; the independence of origin being one of the facts upon which the current theory ' Cf. Tomes, A Manual of Dental Anatomy, 5th ed. Loud
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