152 



Belideus, is much less than in Petaurus. In Belideus the molars oc- 

 cupy a space equal to rather more than two-thirds of that between the 

 incisors and first true molar ; whereas in Petaurus, the four lašt 

 molars occupy more space than that which extends from them to 

 the incisors. There is a corresponding difFerence in the lovver jaw. 

 In Petaurus the molars are very nearly equal in size, w'hereas in Beli- 

 deus they decrease considerably from the first molar to the lašt. In 

 Petaurus, again, there are five molars on each side of the lower jaw 

 opposed to six in the upper jaw, all of wliich are fitted for the masti- 

 cation of the food ; \vhilst in Belideus the molar corresponding to 

 the first on either side of each javv in Petaurus is so small, and its 

 crown is so low, that it cannot be used in mastication. The com- 

 paratively large size of the canines, and the series of small teeth in 

 front of the molars, will also serve to distinguish the species of the 

 present section from the preceding, where the upper margiu of the 

 ramus of the lower jaw somewhat suddenly descends in front of the 

 molars, and the coronoid proccss is comparatively broad. 



" Petaurus sciureus may be regarded as the type of the section Be- 

 lideus, -vvhich will also contain P. flaviventer and P. breviceps. 



" In the third section, which is the subgenus Acrobata of Des- 

 marest, the incisors are ^ ; canines, ^ ; falše molars, ^^i^ ; true mo- 

 lars, J^=36. The incisors resemble those of Belideus; the canines 

 are well-developed, long, pointed, and recurved, placed close to the 

 intermaxillary suture, and even encroaching slightly on the intermax- 

 illary bone. The three falše molars of the upper ja\v have each two 

 fangs, they are compressed, sharply pointed, and viewed laterally, of 

 a triangular form. The first and second are about equal in size, and 

 larger than the third, the apex of \vhich projects beyond the level 

 of the crowns of the true molars. Between the first and second falše 

 molars on each side there is a narro\v space ; the third is placed 

 close to the true molars ; these as well as those of the under jaw 

 resemble the true molars of Belideus ; there is hovvever one less on 

 each side of both jaws. The incisors of the lower jaw also resemble 

 those in Belideus. Behind these incisors there are t\vo minute teeth 

 on each side, which are followed by two sharply pointed falše molars, 

 the foremost of which is the larger, and the apex of the second is 

 raised above the plane of the true molars. 



" The difference in the form of the falše molar teeth pointed out, 

 together with the reduced number of true molars, the slenderness 

 of the zygomatic arch, and the incurved angle of the lower javv, 

 combined with the imperfect statė of the palate, -svill serve to distin- 

 guish the species of the present section from the preceding. Exter- 

 nally, the P, pygmaus (which is the type of M. Desmarest's sub- 

 genus) may be distinguislied by its distichous tail. 



Petaurus bkeviceps. P. cinerea, lined dorsali longitudinali 

 membrandgue laterali supra nigrescentibus, hac ad latera albd į 

 corpore subtūs sordidė et pallidė cinereo : caudd gracili, ad 

 apicem fuliginosd ; auribtis medioctibus. 



