180 DK. C. J. rOJttSYTH MAJOR OX illOC'EXE SQL"1RB£LS. [Feb. 28, 



that the lamiuated h}i)«!odont molars generally begin tuber- 

 cular, and the tubercular brachydont molars, when worn, become 

 laminated. For instance : the unworn upper and lox^er molars 

 of Castor Jiber show us a some^^■hat tuberculate crown, in which 

 at first sight it is not easy to recognize the well-known lami- 

 nated pattern of the worn tooth of the Bea\'er ; likewise, a much- 

 worn molar of the brachydont Crieefus presents enamel-folds 

 and islets, though, owing to the shallower and Mider valleys, they 

 appear less distinct than in the hypsodont molars, whose valleys 

 are reduced to narrow but deep fissures : so that there is only a 

 gradational difference between tubercular and laminated teeth. 



The molar teeth of Sciuridae are generally represented as tuber- 

 cular. But, in surveying all the known forms, even restricting 

 ourselves to the subfamily Sciii.nnce, we meet with all possible 

 intermediate stages between the decidedly hypsodont molars of 

 Eupetaurus described by Thomas ' and the utmost degree of 

 brachydontism as shown by the molars of the Bornean Bhi~ 

 tlirosciv.ms or the Myoxine-Mke teeth of the group of pigmy 

 SquiiTels. 



First, as to Brachydontism and Hypsodontism. 



The species of Pteromijs, in a restricted sense, are on their way 

 to become hypsodont ; they lead over on the one side to Eu^yetaurus, 

 and on the other to the more or less brachydont Sciuro2)te>'i 

 (Lneludtng Pteromi/s tephromelas, Giiuth., and P.jy^ueomelas, Giinth., 

 whose molars are quite similar to each other and agree more with 

 the Sciuro2^teri than with Pteromys). 



The African G-round-Squii-rels {Xerus), as well as the Oriental 

 Sciurus berdmorci, Bly., present a curious form of semi-hypso- 

 doutism, inasmuch as the internal moiety of the upper and more 

 or less the external moiety of the lo\^"er molars are more elevated 

 vertically than the external moiety above and the internal below. 

 Con-esponding to the hypsodont part of the molar, we find on 

 the inner side of superior molars a stout and elongate root, on 

 the outer side two smaller and shorter roots. 



A smaU group of Ethiopian Squirrels included in the genus 

 Scii'.n'S (Sc. palliatus. cepapi, jytirroims, coiu/icus. &c.) present a 

 similar semi-hypsodonty, whilst the Moroccan Xervs getidv.s is in a 

 lesser degree semi-hypsodont. 



This greater vei-tical elevation of the inner side of the crown 

 in superior, and of the outer side in inferior molars, though more 

 evident in semi-hypsodont teeth, is, however, by no means 

 limited to them ; we meet with it, although in a lesser degree, 

 in Sciurus vulgaris and its aUies, and even in the still more 

 brachydont Rhifhrosciurus. Ornitlwrhjnchus itself, as shown by 

 one of the figures published by Stewart ■, has the inner side 

 of the superior teeth more elongate than the outer. This cir- 



1 Oldfield Thomas. "Ou Eupetauftn, a new form of Flying-Squirrel from 

 Kashmir," Journ. As. Soe. Bengal, vol. Ivii. ii. no. 3. 1888, pp. 2.56-260. 



- Ch. Stewart, " On a specimen of the true teeth of Ornithorhj/nchui," Mier. 

 Journ. Tol. sxxiii. u. s. 1891, pl. viii. ('. 



