EVOLUTION OF MOLARS 105 



in many genera ; it is indeed surprising that any of them, in their 

 primitive form, should have Hngered to the present time in any 

 Microtine genus at all. 



It is not possible at present to give a full account of the unworn 

 cheek-teeth of Microtinse, since material suitable for the treat- 

 ment of all the genera does not exist in collections. Nevertheless, 

 we have enough to show the general direction of progress and we 

 can form a fairly clear notion of what has been the course of dental 

 evolution within the group. It is not my intention now to describe 

 all the instances known to me, but merely to give an account of some 

 selected examples illustrating the general argument, premissing, of 

 course, that lam not acquainted with anything that is incompatible 

 with the views here advanced. But before turning to this task 

 it is necessary to say a word about the homologies of tooth cusps. 

 As has long been recognized the inner and outer sides of upper 

 molars correspond respectively with the outer and inner sides of 

 lower molars. Fleischmann ^ went a step further and asserted 

 that the anterior and posterior ends of upper molars are respec- 

 tively homologous with the posterior and anterior ends of lower 

 molars ; so that in his view a lower molar is the completely inverted 

 image of an upper one. Such a conception of the molars if well 

 founded demolishes the homologies of cusps recognized by the 

 supporters of the Tritubercular Theory, and it has been contested 

 by Osborn.2 Forsyth Major ^ said, however, in reply to Osborn : 

 " There can be no doubt as to the correctness of Fleischmann's 

 statement, which is easy to verify. A left upper anterior milk 

 tooth of Didelphys, for instance, is at first sight very difficult to 

 distinguish from one of the right lower series. Even in such 

 specialized molars as those of modern Ruminants, in holding side 

 by side a right upper and a left lower molar, or vice versa, what 

 appear to be the mutual homologies are to be traced out even to 

 very small details." Personally I have not the slightest doubt 

 that lower molars are the completely inverted images of upper 

 ones ; and Fleischmann's conception, inconvenient as it may be to 

 workers who have investigated the molars of so many other orders 

 from a different standpoint, has proved to be of the utmost service 

 in working out the evolution of rodent molars. It removes those 

 contradictions which confront us whenever we compare lower 

 molars with upper molars on the basis of the ordinary view of the 

 homologies ; it gives a meaning to many characters otherwise 

 unintelhgible, and it enables us to carry analysis of the crown 

 structure to a point beyond the reach of the more generally 

 accepted theory. 



The most instructive and at the same time the most complex 

 tooth in Microtinae is m-^, and it is better to begin with this tooth, 

 reserving the discussion of the other molars for a later page. 



1 Sitzungsb. Preuss. Akad. Wiss. Berlin, 1891, 2, p. 891. 



2 Bull. Amer. Mus. Nat. Hist., 1892, p. 84. 



3 P.Z.S., 1893, p. 201. 



