264 



MAHSUPIALIA. 



teetli on each side cf the lower jaw. The four 

 posterior molars in each row are true, and bear 

 four pyramidal cusps, excepting the last tooth 

 in the upper jaw, which, as in Ph. Cookii, has 

 only three cusps. In the upper jaw the space 

 between the functional false molars and the in- 

 cisors is occupied by two simple rudimentary 

 teeth, the anterior representing the canine ; but 

 being relatively smaller than in Ph. Cookii, 

 the crowns of the two anterior incisors are rela- 

 tively larger. In the lower jaw the sloping 

 alveolar surface between the functional molars 

 and large procumbent incisors is occupied, ac- 

 cording to M. Fr. Cuvier, by two rudimentary 

 mimiie teeth, as represented in the figure 

 (fig. 88). I have/jiot found any trace of these 



Fig. 88. 



Petaiirus Taguanoides. 



in the two skulls of Pet. Taguanoides examined 

 by me. In Ph. Cookii we have seen that 

 there are three minute teeth in the correspond- 

 ing space ; but these differences would not be 

 sufficient ground to separate genetically the 

 two species if they were unaccompanied by 

 modifications of other parts of the body. 



In Petaurussciurcusand Petauruxjiaviventer 

 the dentition more nearly resembles that of 

 Phulangista vulpina. In the upper jaw the 

 functional molar series consists of five teeth on 

 each side ; the four hinder ones being, as in 

 Pet. Taguanoides, true tuberculate molars, but 

 diminishing more rapidly in size as they are 

 placed further back in the jaw; the hinder 

 tooth has three tubercles, the rest four ; the 

 apices seem to be naturally blunter than in Pet. 

 Taguanoides, 



Between the functional ptu-molar and the 

 incisors there are three teeth, of which the 



Fig. 89. 



Petaurusflaviventer. 



representative of the canine is relatively much 

 larger than in the Pet. Taguanoides; the first 

 false molar is also larger and has two roots ; the 

 second, which is functional in Pet. Tnguanoides, 

 is here very small. The canine is more deve- 

 loped ; the first incisor is also relatively larger 

 and more produced. In the lower jaw the 

 functional series of grinders consists of the four 

 true tuberculate molars only, of which the last 



is relatively smaller, and the first of a more 

 triangular form than in Pet . Taguanoides. The 

 space between the tuberculate molars and the 

 procumbent incisor is occupied by four small 

 teeth, of which the one immediately anterior to 

 the molars is large, compressed, pointed, and 

 has two roots; the remaining three are rudi- 

 mentary and have a single fang; the anterior 

 of these corresponds to the one regarded as 

 canine in the upper jaw. 



Among the species exhibiting this dental 

 formula, viz. 



T 3 3 1 1 



Incisors : canines ; premo- 



11 11 



lars 



33 



molars 



44 



= 40, 



33 ' 44 



are Pet. sciureus, Pet. Jiaviventer, and Pet. 

 mucriirus. 



The Pigmy Petaurist differs from the pre- 

 ceding and larger species, in having the 

 hairs of the tail distichous, or arranged in two 

 regular lateral series like the barbs of a feather, 

 and in having the spurious molars large and 

 sharp-pointed ; and the true molars bristled 

 each with four acute cusps. This tendency in 

 the dentition to the insectivorous character, 

 with the modification of the tail, induced M. 

 Desmarest to separate the Pigmy Petaurist 

 from the rest of the species, and constitute a 

 new sub-genus for its reception under the name 

 of Acrobates.* 



To Mr. Waterhouse, however, is due the 

 credit of having first pointed out that the Pigmy 

 Petaurist had but three true molars on each side 

 of each jaw instead of four. There seems, there- 

 fore, to be better reason for accepting this sub- 

 generic section, although we evidently perceive 

 a transition to this condition in the small size 

 of the hinder or fourth molars in the Sciurine 

 Petaurist and its congeners. 



The description of the dentition of the Pigmy 

 Petaurist in the Regne Animal, besides being 

 defective in this remarkable particular, is not 

 quite exact in other respects. In four adult 

 specimens, two of which were males, and two 

 females with young in the pouch, I find the fol- 

 lowing dental formula to be constant (fig. 90). 



q o -i 1 



Incisors ; canines ; premo- 



33 1 1 



3 } Q O 



lars 1 ; molars : 36. 



33 33 



Fig. 90. 



Petaunts pigmaeus, twice the natural size. 



The three quadricuspidategrinders of the upper 

 jaw are preceded by three large premolars, each 

 of which has two fangs, and a compressed 

 triangular sharp-pointed crown, slightly but pro-. 



* Axpo;, summits, fjatvtu, gradior, as frequentiiig 

 the summits of trees. 



