MAMMALIA. 565 
(porcupines, guinea-pigs and capybara); the Alyomorpha 
or mouse-like forms (rats, mice, and voles); and the 
Sciuromorpha or squirrel-like forms (squirrels, marmots 
and beavers). 
The beavers are confined to Arctogcea and the Aystrico- 
morpha are most abundant in Neogcea. 
OrDER VIII.— Ungulata. 
The order Ungudata has four living sub-orders which’ 
are sharply distinguished from each other and from other 
orders. The labours of paleontologists have brought 
to light a number of extinct forms which are evidently 
allied to the living Ungulata, though in most cases they 
show, as is to be expected, a number of characters in 
common with the more primitive members of other orders. 
Hence the order has been gradually widened till it now 
contains such a variety of types that they have few special 
features incommon. In a general way they are all herbivor- 
ous and adapted for walking upon land on all four limbs. 
The teeth are heterodont and the canines are, as a rule, 
not longer than the incisors or molars, in many cases resem- 
bling in appearance either of these latter, or they may be 
altogether absent. The premolars and molars are large and 
flat, adapted for grinding and crushing rather than cutting. 
The dentition is diphyodont and the first or milk-series 
remains functiorial for a long time, largely assisting the 
permanent series in their long and arduous duties. The 
lower types have the typical eutherian dentition of $343, but 
this is considerably changed in the more specialised forms. 
The limbs are devoted in this order solely to terrestrial 
locomotion, with its single series of motions. Hence the 
clavicles are nearly always absent and the ulna and fibula 
reduced in the higher types. The carpal and tarsal bones 
remain serial only in the lower types, becoming alternately 
interlocked in the higher. There is a tendency throughout 
the order for a reduction in the number of toes, the third 
alone or third and fourth persisting in the higher forms. 
The typical mammalian claws at the end of the digits 
usually become converted into unguz, or hoofs, presenting a 
flat surface to the ground. There can, along with these 
