180 i>K. C. J. roilSYTlI MAJOR ON MIOCENE SQUIRIIELS. [Feb. 28, 



that the lainiuatetl liypsodont molars generally begin tuber- 

 cular, and the tubercnlar brachydont molars, A\]ien worn, become 

 laminated. For instance : the nnworn upper and lower molars 

 of Castor fiber shoM' ns a someA^hat tnbercnlate ci'oa\ n, in \\hich 

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

 nated ])attern of the worn tooth of the Beaver ; likewise, a much- 

 worn molar of the brachydont Crieetus presents enamel-folds 

 and islets, thongh, owing to the shallower and wider valleys, they 

 appear less distinct than in the hypsodont molars, whose valleys 

 are reduced to uarro^^■ but deep fissures ; so that there is only a 

 gradational difference bet^^•een tubercidar and laminated teeth. 



The molar teetli of Sciuridie are generally represented as tuber- 

 cular. But, in surveying all the kno\^'n forms, even restricting 

 ourselves to the subfamily Scmrrnce, 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 Rhi- 

 tlirosciarus or the Myoxine-llke teeth of the group of pigmy 

 Squirrels. 



First, as to Brachydontism and Hypsodontism. 



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

 to become hypsodont ; they lead over on the one side to Eiqietaurus, 

 and on the other to the more or less brachydont Sciuropteri 

 (vaclwdiing Pierorays tepliromelas, Giinth.,and P.plKvomeJas, Giinth., 

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

 the Sciuropteri than with Pteromys), 



The African Ground-Squirrels {Xerus), as well as the Oriental 

 Sciurus herdmorei, 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. 

 Corresponding 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 t\\"o smaller and shorter roots. 



A small group of Ethiopian Squirrels included in the genus 

 iSciurtis (Sc. palliatus, cepapi, pyrrojtus, coiu/ims, &c.) present a 

 similar semi-hypsodonty, whilst the Moi'occan Xerus getulus is in a 

 lesser degree vsemi-hypsodont. 



This greater vertical elevation of the inner side of the crown 

 in superior, and of the outer side in inferior molars, though inoi'e 

 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 allies, and eA'en in the still more 

 brachydont RhifJirosciurus. Oniithorliynchus 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- 



^ Oklfleld Tliuiiiiis. "On Eupcfawm, a new ibrtu of Flying-Squirrel from 

 Kashmir," Journ. As. Soc. Bengal, vol. Ivii. ii. no. 3, 1888, pp. 25(3-200. 



- Ch. Stewart, " Ou a specimen of the true teeth of Ornithorhynchus," Micr. 

 Journ. vol. xxsiii. n. s. 1891, pi. viii. i. 



