1893.] DR. C.J. rORSYXH MAJOR OX MIOCENE S<iL;iJlB,BLS. 205 



eeeds from the Inner side outwards in snpcrior, and froni the 

 outer side inwards in inferior molars. 

 5. The now prevalent transverse arramjenient of cusps or lobes is 

 not tJie primitive condition, but a specialized pattern of the 

 crown. 



The first two poiuts need no discussion, as no biologist of the 

 present day denies them. 1 thirdly maintain that the morebracJiu- 

 dont a molar is, the )nore polybunous it is ; so that change, which in 

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

 ments, goes hand in hand \\\t\\ the gradual progress from brachy- 

 donty towards hypsodonty. 



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

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

 bunous, so that by this statement alone polybuny is pro\ed to be 

 the primitive condition. 



If M'e examine the outer parts of upper and the inner parts 

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

 in Brachj'dontia as compared to Hypsodontia, and in the various 

 stages of Hypsodontia compared together, than does the rest 

 of the crown, especially the inner side in 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 Iowqv molars (\'iz., 

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

 over the corresponding parts of the opposite jaw) ha\e undergone 

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

 molars. These same sides being generally more comjilex than the 

 't/t;ie/- side above and the outer below, we may infer from it that 

 tlie 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 already in 

 molars of Cretaceous Mammalia and in those of Ornithorhijnchus. 



The extreme of this specialized condition is what has been called 

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

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

 Ci'eodonta, and Lemuroidea than in >Sciuriuce, the species of \^'hich, 

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

 present an approach to trituberculism. 



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

 of superior, and the outer side of inferior molars? 



AVe have seen that in perfectly bi'achydout teeOi the outer 

 and inner sides of the molars present the least difference from 

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

 of their cusps ; and that tlie su[)erior molar becomes shortened 

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

 excessive development of some cusps (generally eitiier one or 

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

 are present in such perfectly brachydont teeth as those of Eoscluri 

 or lihithrosciurti.8. The meaning of this process of reduction be- 

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



