156 DK. B. A. BEXSLEY OX THE EVOLUTION 



excepting M. rufus and M. magnus, in the latter of which the anterior ledge has been 

 reduced. In certain fossil Kangaroos, as shown by the descriptions of Ovveu (1B77) and 

 De Vis (1895), ^ greater degree of molar complication is found than is indicated in existing 

 forms. In Frocojitodoii accessory ridges are very numerous and conspicuous. 



Incisors. — In the two lower subfamilies of the Macropodidse the median upper incisors 

 tend to retain the original predominating size-relations and the grasping character found 

 in the smaller Phalange riute, Caloprymnus alone (like the higher Phalangerinaj, Phalanger 

 and Tj-lchosiu'iis) showing the opposite tendency to reduce tlie vertical length of these 

 teeth while flattening their tips in order to adapt them for a cutting function. The 

 members of the brachyodont series also tend to preserve the original relations, although 

 in a lesser degree. In the present group the modifications begun in Caloprymnus are 

 followed tliroughout, these teeth being always of a distinctly cutting nature. The 

 median lower teeth present the lanceolate cutting modification throughout. The upper 

 lateral teeth show successive stages of sectorial elaboration ; but tliei'c is not a j)erfect 

 progression in this respect from the smaller towards the larger forms. Primitive 

 conditions are found in Lagorchestes, Lagostrophus, Petrogale^ Onychogale, certain of the 

 Small Wallabies {31. Eugenii), and also in certain of the Large Wallabies {M. irma and 

 M. Greyi) which appear to be derivatives of M. Eugenii. In these forms, especially 

 Onychogale, the cutting-edges of the upper lateral teeth are comparatively narrow. In 

 the Small Wallabies, with the above exception, the edges of the third upper teeth tend 

 to be elongated. A more primitive condition is found in 31. Coxenl and 31. Billardleri 

 than in 31. Thetidis, 3£. JFUcoxi, and 31. stigmaticus. The same expansion is noticeable 

 in the Large Wallabies, except in the species referred to, and also in the Kangaroos. 

 Even the latter present differences of a somewhat similar kind to tliose in the lower 

 forms — 31. robustus and 31. giganteus, for example, showing a condition of greater 

 expansion than is found in 31. rufus and 31. magnus. It is an interesting fact that 

 in this elaboration of cutting-edges, which is so important a feature of the grazing 

 evolution, the second upper incisors play a much less important I'ole than the third, their 

 edges being at most only slightly expanded. The explanation is probably to be found in 

 the greater freedom of the third teeth, there being no teeth situate behind them to oppose 

 their posterior elongation. 



Reduplication of the edges of the vipper lateral teeth, already referred to in the case of 

 the brachyodont group, is also indicated in the present series, although the tendency 

 is here in the direction of reduction. In such forms as Dendrolagus and Dorcopsis, 

 where no expansion of the edges is indicated, the tips of the teeth are basin-shaped ; 

 while in more specialized forms, such as those of the present series, where the upper 

 internal teeth cease to act as stops for the lower and become definite cutting-organs, 

 reduplication is less apparent on account of the transverse compression. ' It is still 

 indicated, however, by a notching of the posterior or lateral sides of the teeth. In the 

 Kangaroos the notching is absent in the second incisors and barely indicated in 

 the third, while scarcely more primitive conditions are fuund in the Large A^'allabies. 

 It is apparent that in the development of these forms there has been an increase 

 of muscular perfection admitting of a finer adjustment of the incisor teeth. 



