NUMBER OF TOES 



43 



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

 thorough adaptation to the cursorial mode of progression. In 

 the Mammalia the ankle-joint is always 

 what is termed cruro-tarsal, i.e. 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, which are the astragalus and the 

 calcanerim. 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 the tarsus has 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 cuboid as in 

 the Deer. No mammal has more than five 

 toes, and the number tends to become reduced in c\irsorial 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 Mdiuial of Dental Anatoviy , 5th ed. London, 1898. 



Fig. 32. — Anterior aspect of 

 right femur of Ehinoceros 

 {Rhinoceros indicus). x \. 

 h, Head ; t, great trochan- 

 ter ; t', third trochanter. 

 (From Flo\¥er's Osteology.) 



