171 



falls short; and in Bettongia penicillata (fig. 18) it is reduced to the extent of one and 

 a half of the succeeding molars, or to one-fourth of the entire " rnolary series." 



A comparison more closely bearing upon the use to which a sectorial premolar has 

 been applied is that of the relation of its fore-and-aft length with the length of the 

 " diastema" or interval between it and the incisor ; for the jaws of marsupial Herbivora 

 are commonly characterized by length, and those of Carnivora by shortness. 



Long as the premolar is in Dendrolagus dorcocepthalus, the slender jaw is prolonged 

 to as great an extent before it gives exit to the procumbent incisor ; this interval is never 

 less, usually more, than the fore-and-aft length of the trenchant premolar in all Poephaga. 

 In Thylacoleo the interval between the fore border of the homologous lower premolar and 

 the outlet of the incisor's alveolus is one-fourth the fore-and-aft extent of such premolar. 



The contrast between Thylacoleo and herbivorous Diprotodonts, in the proportion of 

 the trenchant premolar of the upper jaw to the succeeding molars (which in the great 

 carnivore are reduced to one, Plate VII. fig. 3, m i, as in Felis), is still more striking 

 and decisive as to the use of such premolar than in the lower jaw. With the predomi- 

 nance of antero-posterior over vertical extent of crown in the trenchant border, and in 

 the proportions of the two roots of the lower one, the resemblance of the premolar of 

 Thylacoleo to that in any poephagous or herbivorous Diprotodont ceases. It has not the 

 parallel ridges and grooves which characterize the homologous tooth in the Potoroos 

 (Hypsiprymnus, Bettongia, Potorous, &c). 



In the upper sectorial premolar of Thylacoleo, the two best marked ridges are the one 

 defining the anterior border (Plate VII. figs. 1-3, z), and the one terminating the inner 

 prominence of the swollen fore part of the tooth (ib. v) answering to the somewhat more 

 developed ridge in the upper carnassial of Machairodus (ib. figs. 15, 16, v). 



The slight outswelling of the base of this ridge (Plate VII. figs. 2, 15, v') I regard as a 

 rudimental homologue of the internal tubercle of the upper carnassial in Felis. Thus the 

 carnassial in Machairodus (Brepanodon) offers an instructive intermediate modification of 

 that tooth between Felis and Thylacoleo. I am the more impressed by the degree of 

 resemblance through adaptive modification of the sectorial premolar in the carnivorous 

 marsupial, seeing the differences that might be expected, as, indeed, some do exist, in 

 homologous teeth, developed for the same office, in two such different routes of derivative 

 modification as are exemplified by the Marsupial and Placental series of mammalian 

 structures. 



One vertical ridge on the outer and broader fore part of the crown (Plate VII. fig. 1, 

 p 4, u) feebly represents the second lobe of the feline carnassial ; it is divided by a shallow 

 vertical depression from the part (ib. z) representing the anterior lobe of that tooth*. 



limbs, in length or curvature of claws, &c., have afforded the " Gattungsmacherei" grounds for Halmaturus, 

 Lagorchestes, Heteropus, Petrogale, Ospliranter, Dendrolagus, Hypsiprymnus, Bettongia, Potorous, Dorcopsis, &c. 



* This structure is better marked in an upper carnassial of Thylacoleo from the breccia-cave, of which photo- 

 graphs of the outer and inner sides were transmitted to me in the series above noted. 



