128 Zoolofjical Society. 



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 

 formula of the dentition as among the Phalangers. The largest 

 species of Petaurus, Pet. Taguanoides, e. g., is almost identical in 

 this respect with the Phalangista Cookii, which M. Fr. Cuvier has 

 therefore classed with the Petauri. Those teeth of Pet. Taguanoides, 

 which are sufficiently developed, and so equal in length, as to exercise 

 the function of grinders, or in other words, the functional series of 

 molares, include six teeth on each side of the upper jaw, and five 

 teeth on each side of the lower jaw. The four posterior molares in 

 each row are true, and bear four pyramidal cusps, excepting the last 

 tooth in the upper jaw, which, as in Phal. Cookii, has only three 

 cusps. In the upper jaw, the space between the functional false 

 molares and the incisors is occupied by two simple rudimentary teeth, 

 the anterior representing the canine, but being relatively smaller 

 than in Phal. Cookii. The crowns of the two anterior incisors are 

 relatively larger. In the lower jaw the sloping alveolar surface be- 

 tween the functional molares and large procumbent incisors is occu- 

 pied, according to M. Fr. Cuvier, by two rudimentary minute teeth : 

 I have not found any trace of these in the two skulls of Pet. Ta- 

 quanoides examined by me. In Phal. Cookii there are three minute 



* First by Dr. Shaw in the NaiuralisCs Miscellany. 



