lf,l DR. B. A. BENSLET ON THE EYOLUTION 



The Bypsodont Series. (Lagorchestes, Lagostroplius, Ouycliogale, Petrogale, 



Macropus.) 



The present group scarcely calls for generic consideration, since the characters hy 

 which the genera are distinguished are for the most part others than those of the 

 dentition. Generic differentiation apj^ears in this case to have resulted from the 

 assumption by the animals concerned of different kinds of environment while still 

 pursuing a grazing habit. 



The main stages of progressive dental evolution concern the species of the predominant 

 genus Macropus. As recently pointed out by de Vis ( i B95 ), the three sections into wliich 

 the latter have been classified by Thomas (1888) — namely, the Small Wallabies, Large 

 Wallabies, and Kangaroos — represent successive phases of herbivorous evolution. It is 

 interesting to notice that, as in the Dasyuridse, Phalangeridse, and Potoroinae already 

 referred to, the successive phases indicate increase in size of the animals as well as dental 

 specialization. 



Progressloe Characters of Molars. — lleference has already been made to the fact that 

 the assumption of an herbivorous habit involves an extensive wearing-down of the molar 

 crowns, rendering necessary a change from such a condition as is seen in the Phalangerinse, 

 where the teeth are short-crowned, and being functional throughout life tend to decrease 

 in size backwards, because those first formed, namely the anterior ones, are longest in 

 use and may advantageously be largest. This change is now seen to include in its 

 entirety (a) reversal of the relative proportions of the teeth, by which they come 

 to increase in size backwards ; (b) bypsodont development of the crowns ; (c) functional 

 replacement of the teeth from behind forwards, the anterior teeth being worn off and 

 shed while the posterior are coming into service. 



As already pointed out, the tendency to reverse the proportions of the molars is 

 indicated, although to a very limited extent, in the Bettongiinoe and Potoroinae. In the 

 brachyodont division of the Macropodinae, where we meet with dental modifications 

 intermediate in extent of specialization between those of the foregoing smaller divisions 

 and the present bypsodont series, the upper teeth are seen to increase gently in size from 

 the first to the third, the fourth being smaller on account of tiie imperfect develop- 

 ment of its posterior lobe, while the lower teeth increase in size backvi'^ards throughout. 

 In the preseat series we find both the upper and lower teeth increasing markedly in size 

 from the first to the fourth. In respect of this character there is little progressive 

 variation, the modification being only carried to such a stage as to provide the adults 

 with a dentition as functional as that of the young. The forms which are highest 

 in other respects, namely those of the Kangaroo section of Macrojms, are at most only 

 .slightly more advanced in this character than the smaller more primitive forms of the 

 same genus. 



With regard to the functional replacement of the molars, it is seen that in the 

 brachyodont series this princijile is scarcely indicated, the first molars being only slightly 

 worn and still very obviously functional when the fovirth molars are fully in place ; so 



