208 DR. C. J. FORSYTH MAJOR OX MIOCEXE SQUIRRELS. [Feb. 28, 



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

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

 iu tlie lower molars. 



This pa]:)er does not pretend to enter into details as to other 

 families and orders. Bnt I tliink it important to state in a few 

 words that this tendency oi' older forms to\\ards a longitudinal 

 arrangement is quite general in Rodents as well as in Creodouts, 

 Lemuroids, and Ungulates. In Sciuridse we have very primitive 

 forms still existing side by side AA'ith those more specialized, so 

 that the transverse arraugeuient 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 another occasion 

 that the structure of the molar form of Lagomorpha is to be traced 

 back from the perfect transverse direction presented by their 

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

 molar approaching closely to those of Pclycodux^ a mannnal from 

 the Lo\\'er Eocene of North America and Egerkingen in 8wit/erland. 

 Avliich has hitherto been considered to be a Lerauroid. In a some- 

 what lesser degree, the Lagomorphan molar tends towards EstJw- 

 ni/,i; considered by Cope ^ to be one of the progenitors of Eodeiitia. 



The intermediate stages are the unworn milk-teeth, premolars 

 and molars, of young Leims, the Miocene Palo'olagus, Layomys, the 

 Pleistocene, Pliocene, and Miocene Myolayus, and the Miocene 

 Layodus. The anterior upper and lo\Aer premolar of Le^ms, the 

 second superior and the anterior inferior premolar of Layomys, the 

 superior pi'emolars and inore or less all the superior molars of 

 Myolayus, as Mell as the inferior anterior premolar of the latter, 

 sho\\', even in adult specimens, a conformation which points 

 unmistakably towards a longitudinal arrangement of partially 

 sharp-edged cusps — these cusps being three longitudinal series 

 separated by two longitudinal grooves in the up])er teeth, and two 

 series with one intermediate longitudinal groove in the lower 

 molars. The difference bet\Aeen the first lower premolar and the 

 other grinding-teeth is very striking, especially iu Myolaytis, for 

 which I refer to a figure from Pilbol's memoir on the 8ansau 

 fauna "'. 



As to other Orders, I must be satisfied to gixe a few instances. 

 Amongst recent Carnivora, the Hubursi, especially Ailaruf!, and 

 partially the Vrsidse themsehes, show unmistakably the longitu- 

 dinal arrangement of their molar cusi)s. In the same direction 

 the milk-teeth of several Orders poiiat significantly, eveii those of 

 modern liumiuants. 



In the Lower Eocene many molars of various Orders tell the 

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

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



^ E. D. Cope, " The Mecbunical Causes of the Development of the Hard Parts 

 of the Mammalia" (Journal of Morphology, vol. iii. Boston, U.S.A. 1889, 

 p. 263). 



- ' Etudes sur les Mamraiferes fossiles de Sansan ' (Paris, 1891), pi. i. fig. 8. 



