MAMMALIA. 565 



(porcupines, guinea-pigs and capybara) ; the Myomorpha 

 or mouse-like forms (rats, mice, and voles) ; and the 

 Sciuromorpha or squirrel-like forms ( squirrels, marmots 

 and beavers). 



The beavers are confined to Arctogoea and the Hystrico- 

 morpka are most abundant in Neogoea. 



Order VIII. — Ungulata. 



The order Ungulata has four living sub-orders which 

 are sharply distinguished from each other and from other 

 orders. The labours of palaeontologists 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 in common. 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 functional for a long time, largely assisting the 

 permanent series in their long and arduous duties. The 

 lower types have the typical eutherian dentition of f Hf, 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 unguae, or hoofs, presenting a 

 flat surface to the ground. There can, along with these 



