208 DE. C. J. FOKSYTH ilAJOE, ON illOCEJS'E SQUIEEELS. [Feb. 28, 



should say, tlie traces of, a primitive arraugemeiit of its tubercles 

 of upper molars in three lougitudiDal series, there being two rows 

 iu the lower molars. 



This paper does not pretend to enter into details as to other 

 families and orders. But I think it important to state in a few- 

 words that this tendencj' of older forms towards a longitudinal 

 arrangement is qidte general in Eodents as well as iu Creodonts, 

 Lenuu'oids, and Ungulates. In Sciuridae we have very primitive 

 fonns still existing side by side with those more specialized, so 

 that the transxerse arrangement does not at first sight appear to 

 be a later transformation. The Lagomorpha are in this respect, 

 as in others, highly instructive, the molars of the living members 

 being very specialized. I hope to show fully on anotlier occasion 

 that the structure of tlie molar form of Lagomt)rpha is to be traced 

 back from the perfect transverse direction presented by their 

 enamel-ridges to a pelycodoid type of molar, that means, to a 

 molar approachiug closely to those of Pcltjcodvs, a mammal from 

 the Lower Eocene of Xorth America and Egerkingen in Switzerland, 

 which has hitherto been considered to be a Lemuroid. In a some- 

 what lesser degree, the Lagomorphan molar tends toM ards Estho- 

 ny.r, cousidered by Cope ' to be one of the progenitors of Eodentia. 



The intermediate stages are the unworn milk-teeth, premolars 

 and molars, of yoimg Lejms, the Miocene PaJaolagus, Lagomys, the 

 Pleistocene, Pliocene, and Miocene M/jolcu/vs, and the Miocene 

 Lagodus. The anterior upper and lo\\"er premolar of Lejms, the 

 second superior and the anterior inferior premolar of Lagomys, the 

 superior premolars and more or less all the superior molars of 

 Myolagus, as well as the inferior anterior premolar of the latter, 

 show, even iu adult specimens, a conformation \A-hich points 

 unmistakably to^vards a longitudinal arrangement of partially 

 sharp-edged cusps — these cusps beiug three longitudinal series 

 separated by two longitudinal grooves in the upper teeth, and two 

 series with one intermediate longitudinal groove in the lower 

 molars. The difference bet^^•een the first lower premolar and the 

 other grinding-teeth is verj' strildng, especially in Myolagus, for 

 which I refer to a figure from Filhol's memoir on the Sansan 

 fauna '. 



As to other Orders, I must be satisfied to give a few iusiances. 

 Amongst recent Carnivora, the Subiu'si, especially AiJurus, and 

 partially the Ursidse themselves, show unmistakably the longitu- 

 dinal arrangement of their molar cusps. In the same direction 

 the milk-teeth of several Orders point significantly, even those of 

 modern liumiuauts. 



In the Lo\\er Eocene many molars of various Orders tell the 

 same tale as to their origin, often in a distinct manner. In favour 

 of my Aiew I refer to the following figures : — First, from Cope's 



' E. D. Cope, " The Mecbanical Causes of the Development of the Hard Parts 

 of the Mammalia" {Joumal of Morphology, vol. iii. Boston, U.S.A. 1889, 

 p. 263). 



^ ' Etudes sur les Mamraiferes fossiles de Sausan ' (Pai-is, 1891), pi. i. fig. 8. 



