1893.J BR. C. J. rOBSYTH MAJOR OX illOCEXE S(iUIRBELS. 205 



ceecls from the inner side outwards in superior, and from the 

 outer sidx inwards in inferior molars. 

 5. Tlie now prevalent transverse arrangement of ciisjis or lobes is 

 not the primitive comlition, but a specialized pattern of the 

 crown. 



The first two points ueed no discussion, as no biologist of the 

 present dav denies them. 1 thirdly maintain that the more brach;/- 

 dont a molar is, the more pol ijbu nous it is ; so that change, which in 

 our case means reduction, simphtication, of the molar-crown ele- 

 ments, goes hand in hand with the gradual progress from brachy- 

 donty towards hypsodonty. 



In the general survey of Sciurine teeth, it has already been shown 

 that the more the iriolars are brachydont, the more they are poly- 

 bunous, so that by this statement alone polybuny is proved to be 

 the primitive condition. 



If we examine the outer parts of upper and the inner parts 

 of lower molars, we see that they present much less variation 

 in Brachydontia as compared to Hypsodontia, and iu the various 

 stages of Hypsodontia compared together, than does the rest 

 of the crown, especially the inner side iu upper and the outer 

 side in lower molars. It therefore at once strikes us, that 

 the outer side of upper and the inner side of lower molars (viz., 

 those parts which, when the jaws are at rest, are protruding 

 over the corresponding parts of the opposite jaw) have uudergone 

 the least modifications, that they are the more stable elements of 

 molars. These same sides being generally more complex than the 

 inner side above and the outer below, we may infer from it that 

 the complex condition is the primary one, and that the reduced, 

 simplified state of the inner side above and the outer side below is 

 a specialized condition, the beginning of which we see alread\- in 

 molars of Cretaceous Mammalia and iu those of Ornithorhtj nchu^. 



The extreme of this specialized condition is what has been called 

 tritubercuhsm, and considered to be a primitive pattern of Euthe- 

 riau molars. It is not more primitive in Uugulata, Condylarthra, 

 Creodonta, and Lemuroidea than iu ,Sciurince, the species of which, 

 when there is only one cusp on the inner side of upper molars, 

 present an approach to trituberculism. 



Now, what is the meaning of this reduction on the inner side 

 of superior, and the outer side of inferior molars ? 



We have seen that in perfectly brachydont teeth the outer 

 aud inner sides of the molars present the least difference from 

 each other in longitudinal extension, as well as iu the number 

 of their cusps ; and that the superior molar becomes shortened 

 on its inner side, as well as the inferior on its outer side, by the 

 excessive development of some cusps (generally either one or 

 three, rarely two, in Sciurhue) at the expense of others, which 

 are present in such perfectly brachydont teeth as those of Eosciuri 

 or RhitJirosciurus. The meaning of this process of reduction be- 

 comes obvious, when we consider that the internal cusps of superior, 



