13 



rudimental teeth on each side represent the small spurious molar, 

 and small canine of the upper jaw ; and anterior to these, there is one 

 very small and one very large and procumbent incisor on each side. 



The constant teeth in this group are the *-=| true molares, and the 

 j^ incisors. The canines j^ are constant in regard to their pre- 

 sence, but variable in size ; they are always minute in the lower jaw. 

 With respect to the spurious molares, J^, they are always in contact 

 with the true grinders, and their crowns reach to the same grinding 

 level ; sometimes a second spurious molar is similarly developed as in 

 the Phal. Cookii, and as in all the flying Phalangers, or Petaurists, but 

 it is commonly absent or replaced by a very minute tooth, shaped 

 like a canine : so that between the posterior spurious grinder and 

 the incisors we may find three teeth, of which the posterior is the 

 largest, as in Phal. Cookii, or the smallest, as in Phal. cavifrons ; 

 or there may be only two teeth, as in Phal. ursina and Phal. 

 vulpina, and the species, whatever that may be, which Fr. Cuvier 

 has selected as the type of the dentition of this Genus. 



In the lower jaw similar varieties occur in these small and unim- 

 portant teeth ; e. g. there may be between the procumbent incisors 

 and the posterior false molar, either four teeth, as in Phal. Cookii ; or 

 three, as in Phal. cavifrons ; or two, as in Phal. ursina, Phal. ma- 

 culata, Phal. chrysorrhoos ,• or lastly, one, as in Phal. vulpina, and 

 Phal. fuliginosa . 



The most important modification is presented by the little Phal. 

 gUriformis of Bell, which has only three true molares on each side of 

 each jaw. 



Genus Petaurus. 



There are many species of Marsupials limited to Australia, and 

 closely resembling or identical with the true Phalangers in their 

 dental characters and the structure of the feet. I allude to the Pe- 

 taurists or Flying Opossums ; these, however, present an external 

 character so easily recognizable, and influencing so materially the lo- 

 comotive faculties, as to claim for it more consideration than the mo- 

 difications of the digits or spurious molares, which we have just been 

 considering in the Phalangista. A fold of the skin is extended on 

 each side of the body between the fore and hind legs, which, when 

 outstretched, forms a lateral wing or parachute, but which, when 

 the legs are in the position for ordinary support or progression, is 

 drawn close to the side of the animal by the elasticity of the subcu- 

 taneous cellular membrane, and then forms a mere tegumentary 

 ridge. These delicate and beautiful Marsupials have been separated 

 generically from the other Marsupials under the name of Petaurus* : 

 they further differ from the Phalangers in wanting the prehensile 

 character of the tail, which in some species of Petaurus has a general 

 clothing of long and soft hairs, whilst in others the hairs are arranged 

 in two lateral series. 



Now in the Petaurists there is as little constancy in the exact 



• First by Dr. Shaw in the Naturalisi's Miscellany. 



