KANGAROOS. 389 



only is retained on each side of each jaw in the Potoroos as in the 



Koala. 



The dental formula of Hypsiprymnus, the generic name of the 



Potoroos, is 



4—4 

 3 3 J 1 [ 1 ^ • 



Incisors — - ; canines -3- ; premolars -— ; molars jzr4 : = 30. 



The two anterior incisors are longer and more curved, the 

 lateral incisors relatively smaller than in the Koala. The pulps 

 of the anterior incisors are persistent. The canines are larger than 

 in the Koala ; they always project from the line of the intermaxillary 

 suture ; and while the fang is lodged in the maxillary bone, the 

 crown projects almost wholly from the intermaxillary. In the large 

 Hypsiprymnus ursinus the canines are relatively smaller than in the 

 other Potoroos, a structure which indicates the transition from the 

 Potoroo to the Kangaroo genus. In the skeleton of this species in 

 the Leyden Museum, the canines have a longitudinal groove on the 

 outer side. 



The characteristic form of the trenchant premolar has just been 

 alluded to : its maximum of development is attained in the arboreal 

 Potoroos of New Guinea fHypsiprymnus ursinus and Hyps, dorcoce- 

 phalusj, in the latter of which its antero-posterior Extent nearly 

 equals that of the three succeeding molar teeth. In all the Potoroos 

 the trenchant spurious molar is indented, especially on the outer side 

 and in young teeth, by many small vertical grooves. The true 

 molars each present four three-sided pyramidal cusps ; but the 

 internal angles of the two opposite cusps are continued into each 

 other across the tooth, forming two angular or concave transverse 

 ridges. In the old animal these cusps and ridges disappear, and the 

 grinding surface is worn quite tlat. 



Genus Macropus. Kangaroos. — In the genus Macropus (fig. 8,) 

 the normal condition of the permanent teeth may be expressed as 

 follows : — 



Incisors — ; canines — ; premolars — ; molars — : = 28. 



1— I ' 0—0 ' i 1—1' 4—4 



The main difference, as compared with Hypsiprimnus, lies in 



