194 DE. c. J. Foiisrin majou ou miocene squierels. [Feb. 2S, 



Similar remarks apply to the skull. As to molar-forms, I draw 

 attention to two of the most cui-ious in this respect, those of 

 iSciuro/derus Jiorsjleldi, Waterh. (Plates A'lII. and IX. fig. 21), and 

 Sciuropt, pearsoni, Gray (Plates YIII. and IX. fig. 20). The upper 

 molars of both of these show, on the inner side, t\\o crescents 

 or cusps, with an interveuiug depi'ession — a conformation which 

 in recent adult Sciuromorpha is quite unique, but is met with in 

 the Eocene genera ScmroiJcs, Iscltijromys, Fseudosciurus. Pseuclo- 

 sciurKS, moreover, though being soDiewhat more brachydout than 

 Sciuropt. pearsoni, has in common with it the sculpturing of the 

 enamel, as well as the pattern of the crown in general, there being 

 six cusps in the upper molars, which in the living species show a 

 tendency to form crests. Sciuropti'rus liorsfieldi shoMS other 

 relations to Sciuroides, besides those already mentioned. 



Now, how are we to account for this astonishing variety of 

 forms, wliich show this group in a new and unexpected light? 

 The brachydontism for one thing proves that the Sciuropferi are old 

 forms, and we have seen that they made their appearance certainly 

 in the Middle Miocene in a form which has undergone very little 

 change up to the present day, aud that nearly related forms 

 appear in the Oligoceue of Prance aud England, and in the Miocene 

 of Oregon. Moreover, the nearest related fossil forms, of two of 

 the species at least, are Eocene. In stating this, I do not pretend 

 to say that Sciuroides and Pseudosciunis were Plying-Squirrels, 

 but only that there is a near relationship between them and the 

 two Sciuropteri mentioned. 



Now, as to the genetic relation of the Hying to the ordinary 

 Squirrels, there are three suppositious possible : — 



(1) The most obvious would be to admit that the Plyiug- 

 Squirrels have e^•olved from various species of nou-flying Sciuro- 

 morpha. In fa'\"0ur of this supposition it coidd be said that, 

 considering Plying-Squirrels to be more specialized, they have 

 apparently evolved from less specialized forms, and that the more 

 fa\ourable conditions in escaping their enemies might account for 

 the surnval of such forms as Scinropt. Iwrsfieldi and Sc. pearsoni, 

 Avhose nearest relations were largely represented in the older 

 Tertiaries. In support of a supposed independent evolution of 

 the species of ScluropU'ri from several species of Sciitri, the fact 

 might be adduced that there exists some similai'ity in the dentition 

 of an Oriental ficiuroptenis (Sc. Jimhriatus, Grr.) with several 

 Oriental Sciuri (Sc. jirci'osii, e. g.) ; and the same might be said 

 with regard to the Nearctic Sciuropterus volucella. Pall, sp., as 

 compared Anth several Nearctic Sciuri, 



(2) A second possible supposition would be the re^'erse of the 

 first. Por one reason or other, a patagium might have proved 

 disadvantageous, and the Sc. Jmdsonius might be the descendant of 

 such a form as Scvuropt. volucella, as Sciurus prevosti aud other 

 Oriental Sciuri might have sprung from some SciuropAerus Jimhri- 

 atus. In favour of this supposition, the fact might be adduced 

 that the species of Sciuropteri and Pterotmjs, taken together, are 



