MAMMALIA. 563 
to the Eocene. Haditherium also had a diphyodont dentition 
and the pelvic-girdle and hind-limb were not so reduced as 
in present-day species. The Svvenéa are usually regarded as 
having been derived from very generalised terrestrial herbi- 
vores, approximating to the lowest Ungu/ata, 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, little 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 Mammaha. ‘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 dastema and a number of premolars 
and molars, which are often reduced from the $% of the 
rabbit to 2-3, 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. 
