MAMMALIA. 563 



to the Eocene. Halitherium also had a diphyodont dentition 

 and the pelvic-girdle and hind-limb were not so reduced as 

 in present-day species. The Sirema are usually regarded as 

 having been derived from very generalised terrestrial herbi- 

 vores, approximating to the lowest Ungulata, but there is little 

 direct evidence at present for such a view. They are a 

 primitive and much modified order, in these respects resem- 

 bling the two preceding orders, and though there is no 

 question that they are descended from terrestrial eutherian 

 mammals, Httle more can be said. 



Order VII. — Rodentia. 



The rabbit has already been described as a typical 

 mammal, and, except in respect to their peculiar dentition, 

 the Rodentia, as a whole, are a group with habits and 

 structure which apparently approximate to those of the 

 primitive Eutherian Mammalia. Thus they are all of small 

 size, mainly terrestrial, though some are arboreal, usually 

 plantigrade, with little or no reduction in the number of 

 toes, each of which carries a scratching claw. The orbit is 

 never completely encircled by bone, the clavicles are always 

 present though often reduced, and there is often a third 

 trochanter. 



But the most distinctive character of the order is the 

 dentition. The canines are always absent and the incisors 

 are reduced in the majority of cases to two in each jaw. 

 These grow perpetually from persistent pulps, and as the 

 enamel or hardest portion of the tooth is only present on 

 the outer surface, the wear of upper and lower teeth on 

 each other produces a sharp chisel-like edge. These teeth 

 are used, in the majority of cases, for other purposes in 

 addition to that of obtaining food. The teeth are succeeded 

 by a large space or diastema and a number of premolars 

 and molars, which are often reduced from the f-| of the 

 rabbit to y-f , or even, in exceptional cases, to §-f . 



The molars vary much in character, but are always flat 

 and worn on the surface, exposing complex enamel-ridges. 

 In order that the incisors may have free play, the condyle is 

 freely movable in the glenoid cavity and there is no post- 

 glenoid process. 



