9. BEITONGIA. 105 



on the upper side longer than those on the lower, and generally 

 darker in colour, forming a more or less distinct crest. 



Skull short, stout and strong, with a very short muzzle. Inter- 

 orbital space broad and flat, its edges square or slightly rounded. 

 Anterior palatine foramina very short, extending as a rule only from 

 about the middle of i.^ to the middle of the canine. Palate with a 

 single pair of very large vacuities between the molars, only bordered 

 behind by a narrow bar of bone, and often confluent in the middle 

 line. BuU» very variable in size, generally much, but sometimes 

 only slightly swollen. Lower jaw short, thick, with a marked 

 convexity belo-\y. 



Dentition :-I. i^«, C. J, P. l^^^^, M. >^|4^J»x2=34. 



Incisors much as in JEpyprymnus. Canines present, thick, 

 rather shorter than the posterior incisors. P.^* (PI. XIII. figs. 

 6-10) very long, powerful and trenchant, with but little trace 

 of internal ledge, but with a small postero-internal talon ; its 

 two surfaces each With from seven to fifteen distinct oblique grooves 

 passing downwards and backwards : the inner ones efiaced by wear 

 in old individuals. Deciduous p." similar to p.*, but shorter and 

 smallel:. Molars quadrangular and quadritubercular, markedly 

 decreasing in size backwards ; m.' much the smallest of aU. 



Range. Australia (except the extreme north) and Tasmania. 



The members of this genus alone among terrestrial mammals have 

 their tails prehensile, using them for carrying grass, sticks, or other 

 objects, which, as shown in Gould's figures, are held by the tail 

 being twisted downwards round them. 



The four species, although very distinct and easily definable from 

 each other by their cranial and dental characters, are yet extra- 

 ordinarily similar to each other externally. So much is tliis tho 

 case that in the following synopsis it has been found impossible to 

 do more than point out the most obvious distinguishing characters ; 

 characters that are, however, so slight and so variable that absolute 

 reliance can be placed on no determination that does not depend 

 upon an examination of the skull. 



* Normally; but the members of this genus are apparently only now in 

 process of losing their fifth molars, and present therefore the remarkable con- 

 dition of retaining such an ancient and generalized oharafter as the possession 

 of more than four molars, while at the same time they hffve the comparatively 

 highly specialized characters of the other members of the present family. 

 Waterhouse (N. H. Mamm. i. p. 206) notices the occurrence of a fifth molar 

 in a specimen of B. lemmri, but in addition I have been able to find one other 

 instance of its occurrence on one side of the upper jaw, two instances on both 

 sides, and, finally, one instance, in B. ctmicul/us, of its occurrence on both sides 

 of both jawsj this animal having therefore a total of 20 molars. In several 

 cases the extra teeth are still beneath the bone, and appear as if they never would 

 have been out, a state of things quire paralleled in the now progressing reduction 

 of the number of human molars, by the frequent non-eruption after develop- 

 ment of the so-called " wisdom-teeth " (m.'). On the other hand m.'' is itself 

 often aborted in Bettongia, there being then only three molars. 



