290 ANATOMY OF VERTEBRATES. 



like a canine : thus, in the upper jaw, between the posterior or 

 functional premolar, p 3, and the incisors, z, we may find three 

 teeth, as in Phal. Cookii, or two teeth as in Phal. vulpina, the 

 first being the canine, c. In the lower jaw similar varieties occur 

 in these small and unimportant teeth : e. g. there may be between 

 the procumbent incisors and the posterior premolar, either three 

 teeth, as in Phal. Cookii; or two, as in Phal. ursina; or one, as in 

 Phal. vulpina. The most important modification is presented by 

 the little Phal. gliriformis and Petaurus (Acrobates) pygm&us, 

 fig. 219, vol. ii., which have only three true molars on each side 

 of each jaw. These minor modifications are unaccompanied by 

 any change of general structure or of habit, whilst those teeth 

 which most influence the diet are constant. 



The absence of functionless premolars and of lower canines is 

 constant in the Koala (Phascolarctos, fig. 221, vol. ii.). The 

 molars are proportionally larger than in the Phalangers : each is 

 beset with four three-sided cusps, the outer series in the upper 

 teeth being the first to wear down ; those in the lower jaw are nar- 

 rower than in the upper ; there is also the rudiment of a i cingu- 

 lum.' The premolars are compressed, and terminate in a cutting 

 edge. The small canine is situated close to the premaxillary suture. 



The dental formula of the Potoroos {Hypsiprymnus) is — 

 • 3-3 ii i-i 4.4 . n m 



The anterior of the upper incisors are longer and more curved 



than the lateral ones, and 

 their pulps are persistent. 

 The canine is larger than 

 in the Koala ; it is simi- 

 larly situated. In the large 

 Hypsiprymnus ursinus the 

 canines are relatively 

 smaller than in the other 

 Potoroos, a structure which 



Dentition of Hypsiprymnus murinus. m ' ( m 



indicates the transition 

 from the Potoroo to the Kangaroo genus. The single premolar, 

 p 3, has a peculiar trenchant form; its maximum of develop- 

 ment is attained in the arboreal Potoroos of New Guinea ; 

 in Hypsiprymnus dorcocephalus, e.g. 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, m l, 2, 3, 4, have large 





