considered as a zoological Character. 457 



shall hereafter call them, the Simiadse, are not the only mam- 

 mals which exhibit the peculiar conformation of the organs 

 of locomotion and prehension here described. There is a 

 small family of marsupials which possess the same character 

 of having opposable thumbs on the posterior extremities only, 

 and which, in spite of considerable variety in the dentition of 

 the different genera, form a very natural group, which I cha- 

 racterised, some years ago, in a paper read before the Linnaean 

 Society, and of which I shall now recapitulate the most im- 

 portant affinities. The group comprehends the genera Di- 

 delphys, Cheironectes, Balantia, Phalangista, Petaurus, and 

 Phascolarctos, together with a new genus, Pseudocheirus *, 

 which I have found it necessary to separate from Phalangista 

 as at present constituted. All these genera, besides the cha- 

 racters proper to them as marsupial and pedimanous animals, 

 agree in their nocturnal habits, omnivorous appetites, and 

 arboreal lives. It has been usual hitherto to separate the 

 Phalangers and Petaurists from the Opossums, and even from 

 the Koalas, on the supposition that the modifications observ- 

 able in the dental system of these genera betokened a differ- 

 ence of regimen. Thus, we have had the various families f of 

 EntomophagaJ, Car6phaga§, Phyllophaga ||, Frugivora-j-, &c. 

 all claiming the rank of natural groups, though founded solely 

 upon this presumption. Such cl priori reasoning, however, 

 should be very sparingly used in a science which depends 

 entirely upon observation ; and, in the present instance, the 

 experience which I have had in studying the habits and ap- 

 petites of the numerous species of all those genera which have 

 been from time to time exhibited in the Zoological Society's 

 gardens convinces me that there is little or no difference, in 

 this respect, between the Opossums and Phalangers, but that 



* It is onl'y necessary to observe, that this genus is distinguished by 

 a peculiar formation of the anterior extremities, the fingers being di- 

 vided into two groups, as in Phascolarctos ; not, indeed, opposable to 

 one another, but sufficiently separated to facilitate the act of prehension 

 in moving among the branches of trees. It is separated from Pha- 

 langista, and includes P. Cookii and P. gliriformis. Before noticing the 

 important character upon which I have founded this new genus, I 

 could never understand why Mr. Bell, in his excellent paper on the 

 latter species in the Linncean Transactions, spoke of it as having op- 

 posable thumbs on the fore feet as well as on the hind. The fact is, 

 however, that it has two thumbs, if I may be allowed a slight latitude 

 of expression, on the anterior extremities ; which, though not exactly 

 opposable to the other fingers, are still sufficiently separated from them 

 to have the general appearance of being so. 



f Cuv. Reg. Anim., i. 175. % Desm. Mam., 262—275. 



§ Latr. Fam. Nat., 53. || Less. Man. des Mam., 217. 



4. F. Cuv. Dents des Mam., 125. 



