194 Dii. c. J. roiisYxu majoe on miocene SQumaELS. [Feb. '2b, 



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

 atteution to two of the most curious in this respect, those of 

 (:>civ.ropterus horsfieldl, AVaterh. (Plates A'lll. aud IX. fig. 21), aud 

 Sciv.ro^d. pearsoni. Gray (Plates VIII. aud IX. fig. 20). The upper 

 molars of both of these show, on the inner side, two crescents 

 or cusps, with an intervening depression — a conformation which 

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

 the Eocene genera Sciurouh'S, Isclnjromys, Pseud osciums. Pseudo- 

 sciurv.s, moreover, though being somewhat more brachydoiit than 

 iSciurojjt. 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, wliich in the living species show a 

 tt;ndency to form crests. Sciuropterus Iwrsfieldl sho\\s other 

 relations to Sciuroides, besides those already mentioned. 



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

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

 The brachydontism for one thmg proves that the Sc'inropierl 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, and that nearly related forms 

 appear in the Oligocene of Prance and England, and in the Miocene 

 of Oregon. Moreover, the nearest related fossil forms, of t\\o of 

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

 to say that Sciuroides and Pseudosciarus \\ere Plying-Squirrels, 

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

 two Sciuropteri mentioned. 



Xow, as to the genetic relation of the flying to the ordinary 

 fSquirrels, there are three sujjpositious possible : — 



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

 iSquirrels have e\ olved from Aarious species of nou-flying Sciuro- 

 morpha. Li favour of this supposition it could be said that, 

 considering Plying-JSquirrels to be more specialized, they have 

 apparently e\ olved from less specialized forms, and that the more 

 fa\ourable conditions in escaping their eneinies might account for 

 the survival of such forms as Scivrojit. liorsfieUi aud Sc. ^icarsoni, 

 whose nearest relations were largely represented in the older 

 Tertiaries. In support of a supposed independent evolution of 

 the species of Sdv.roplcri from several species of Sciuri, the fact 

 might be adduced that there exists some similarity in the dentition 

 of an Oriental Sciuropterus (Sc. fimhrlatus, Gr.) with several 

 Oriental Hdurl (,Sc. jirevosti, e. g.) ; and the same might be said 

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

 compared w ith several Xearctic Sciuri. 



(2) A second possible supposition would be the reverse of the 

 first. For one reason or other, a patagium might have ])roved 

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

 such a form as Sciuropt. volucella, as ''^cinrus pirevosti and other 

 Oriental Sciuri might have sprung from some Sciuro/Aerus firnhri- 

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

 that the species of Sdaropteri and PkronojH, taken together, are 



