NUMBER OF TOES 



43 



the hind-limbs must have preceded the fore- limbs in their 



thorough adaptation to the cursorial mode of progression. In 



the Mammalia the ankle-joint is always 



what is termed eruro-tarsal, i.e. between 



the ends of the lind>-bones and the proximal 



row of tarsals; not in the middle of the 



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 tlian two 1 tones in the proxi- t' 



mal row, which 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 



bones differ widely. Below these is the 



navicular, regarded as a, centrale. The 



distal row of tlie tarsus luis four bones, 



three cuneiforms and a cuboid. Eeduction 



is effected by the soldering together of 



two cuneiforms as in the Horse, by the 



fusion of the navicular and cul)oid as in 



the Deer. No mammal has more than five 



toes, and the number tends to become reduced in cursorial animals 



(Eodents, Ungulates, Kangaroos). 



Teeth. — The 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 tissue — the 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 which 

 they are later so intimately connected ; the independence of 

 origin being one of the facts upon which the current theory 

 ^ Cf. Tomes, A MamuU i>f Denial Anatomy, 5tli ed. Loudon, 1898. 



Fig. 32. — Anterior asjiect ol' 

 right fennir of Rhinoceros 

 {lihinnceros iiuUcus). x ^,. 

 A, Head ; t, great trochan- 

 ter ; t', third trochanter. 

 (From Flower's Osteoloiji/.) 



