OCR Text |
Show 194 DR. C. J. EORSYTH MAJOR ON MIOCENE SQUIRRELS. [Feb. 28, Similar remarks apply to the skull. As to molar-forms, I draw attention to two of the most curious in this respect, those of Sciuropterus horsfieldi, Waterh. (Plates VIII. and IX. fig. 21), and Sciuropt. pearsoni, Gray (Plates VIII. and 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 Sciuroides, Ischyromys, Pseudosciurus. Pseudosciurus, moreover, though being somewhat more brachydont 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. Sciuropterus horsfieldi shows other relations to Sciuroides, besides those already mentioned. N o w , how are w e to account for this astonishing variety of forms, which show this group in a new and unexpected light? The brachydontism for one thing proves that the Sciuropteri are old forms, and w e 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 France and 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 Pseudosciurus were Flying-Squirrels, but only that there is a near relationship between them and the two Sciuropteri mentioned. N o w , as to the genetic relation of the flying to the ordinary Squirrels, there are three suppositions possible:- (1) The most obvious would be to admit that the Flying- Squirrels have evolved from various species of non-flying Sciuromorpha. In favour of this supposition it cotdd be said that, considering Flying-Squirrels to be more specialized, they have apparently evolved from less specialized forms, and that the more favourable conditions in escaping their enemies might account for the survival of such forms as Sciuropt. horsfieldi and Sc. pearsoni, whose nearest relations were largely represented in the older Tertiaries. In support of a supposed independent evolution of the species of Sciuropteri from several species of Sciuri, the fact might be adduced that there exists some similarity in the dentition of an Oriental Sciuropterus (Sc. fimbriatus, Gr.) with several Oriental Sciuri (Sc, prevosti, e. g.); and the same might be said with regard to the Nearctic Sciuropterus volueella, Pall, sp., as compared with several Nearctic Sciuri. (2) A second possible supposition Mould be the reverse of the first. For one reason or other, a patagium might have proved disadvantageous, and the Sc. hudsouius might be the descendant of such a form as Sciuropt, volueella, as Sciurus prevosti and other Oriental Sciuri might have sprung from some Sciuropterus fimbriatus. In favour of this supposition, the fact might be adduced that the species? of Sciuropteri and Pteromys, taken together, are |