II NUMBER OF TOES 43 
the hind-lhmbs must have preceded the fore-limbs in their 
thorough adaptation to the cursorial mode of progression. In 
the Mammaha the ankle-joit is always 
what is termed cruro-tarsal, 7.e. between 
the ends of the limb-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 than two bones in the proxi- 
mal row, which are the astragalus and the 
caleaneum. 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. Reduction jy 39. anterior Wetectiot 
is effected by the soldering together of tight femur of Rhinoceros 
two cuneiforms as in the Horse, by the Cee eles ee eee 
, ad, ¢, great troche 
fusion of the navicular and cuboid as in _ ter; “, third trochanter. 
the Deer. No mammal has more than five 7° "lowers Oveoleuy) 
toes, and the number tends to become reduced in ecursorial animals 
(Rodents, 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, 4 Manual of Dental Anatomy, 5th ed. London, 1898. 
