1286 



CLASS MAMMALIA. 



deciduous, the latter being invariably preceded by a milk-tooth. The 

 true molars have either four tubercles or two transverse ridges, and 

 frequently an anterior talon. The mandible has a deep pit and per- 

 foration in the masseteric fossa. The pectoral limb is always more 

 or less markedly shorter than the pelvic. The manus is provided 

 with five subequal digits; but in the pes (fig. 1157) the hallux is 

 generally absent, the second and third digits being 

 very minute and enclosed in a common integument 

 {syndactylous), while the fourth is greatly enlarged, and 

 forms the main base of support. The majority of 

 forms progress by making enormous leaps by means 

 of their powerful hind-limbs ; but a few, like Dendro- 

 lagus of New Guinea, are arboreal. This family may 

 be divided into three subfamilies. In the first, or 

 Hypsiprymnodontince, there is a distinct hallux, and 

 the dentition closely resembles that of the Phalan- 

 geridce ; the fourth premolar being small, obliquely 

 grooved, and directed inwardly at its anterior border. 

 In the small existing Hypsiprymnodon there is no tooth 

 between the lower incisor and the fourth premolar ; 

 but in a large extinct form from the Pleistocene of 

 New South Wales, described under the name of Tri- 

 dis, there is a minute tooth behind the lower incisor 

 corresponding to the tooth in the PAa/angeridce com- 

 monly reckoned as the representative of the canine. 

 This genus, therefore, forms an important link between 

 the last-named and the present family. In the second 

 subfamily, or Potoroince, the hallux is absent; the 

 first upper incisor is narrower and longer than either of 

 the others (fig. 1158); there is always an upper can- 

 ine ; the fourth upper premolar is elongated and secant, and usually 

 with the grooves strongly marked ; while the true molars are tuber- 

 culate, with the fourth smaller than the third. The Kangaroo-rats 

 are divided into the genera Potorous {Hypsiprymnus), Bettongia, 

 and sEpyprymnus ; the latter being represented in the Pleistocene 

 of New South Wales by remains of the existing species. 



The third subfamily, or Macropince, is distinguished from the last 

 by the following characters : The cutting-edges of the upper incisors 

 form nearly a straight line (fig. 1159); the upper canine is usually 

 either absent or very small ; and the fourth premolar, which may be 

 shorter or longer than the first true molar, has either an inner basal 

 ridge or lobe. In the existing genus Macropus (Kangaroos) the 

 fourth upper premolar has a sharp cutting outer edge, and an inner 

 ridge or tubercle ; and this tooth in both jaws may be either longer 

 or shorter than the first true molar. The two rami of the mandible 



Fig. 1157. — 

 Dorsal aspect of 

 the right pes of 

 Macropus Ben- 

 netti. Reduced. 

 (After Flower.) 



