OCR Text |
Show 190 DR. 0. J. tfORSVTH MAJOR ON MIOCENE SQUIRRELS. [Feb. 28, " middle-sized " Oriental Squirrels. Division y, with Sc. vulgaris and Sc. syriacus, distinguished from each other by slight characters of the first superior premolar (p1). Division 3 contains most of the American Squirrels. The third subgenus c. is Tamias, which I cannot separate from the genus Sciurus. The skull of the Pigmy Squirrels is very remarkable, a fact to which attention has heen drawn by Miiller and Sehlegel, as well as by 0. Thomas. The anterior zygomatic root is a perfectly vertical narrow septum, and is placed so much backwards (above the second molar) that the facial part of the cranium appears very elongate, whilst the frontal region is broader relatively than in any other Sciuromorpha. The orbita is formed by an almost circular bony ring. The dentition, of which I give the figures (Plate X . fig. 10, Plate X I . figs. 6, 7) is unique amongst Sciuromorpha, and can, as I have said before, be compared only with that of some Myoxi. In the upper jaw there are either one or two premolars, and one in the lower jaw. The posterior upper premolar (p1) and the lower premolar (pt) are of minute size as compared to the homologous teeth in other Sciuromorpha, sometimes scarcely one-third of the size of the molars. This is another feature they have in common with certain of the Myoxi. The pattern of the crown differs from that found in the Sciuromorpha generally, in presenting only three complete transverse crests in the upper molars instead of four, and three in the lower molars. The third crest, counting from front to back, of Sciuromorphous molars is very reduced in these pigmy Squirrels, sometimes scarcely more than a minute cusp. A further peculiarity of these molars is the large development of the anterior transverse valley of both superior and inferior molars, as compared to the usual Sciuromorphous type, sometimes almost equalling that of the posterior valley. This last, owing to the partial suppression of the third crest (Eiitimeyer's Nachjoch), occupies the area of the median as well as that of the posterior transverse valley in the tooth of Sciuromorpha. These characters of the teeth and the skull of the Pigmy Squirrels sufficiently justify the creation of a separate subfamily for them. IV. Description of the Fossil Species. The remains of two species of Squirrels are representatives, the one (1) Sciurus spermophilinm, Dep.1, of the Sciurus vulgaris-type, the other (2) Xerus grivensis, n. sp., of the Zfr«s-type, which both types have been so fully discussed before, that I need scarcely enter into further details. 1 Ch. Deperet, "Recherches sur la succession des Fauties de Vertebres miocenes de la Vallee du Rhone," Arch, du Mus. d'Hist. Nat. de Lvon, t. V. (Lyon, 1887), pp. 108-110, pi. xiii. figs. 14, 14 a. Id., "La Faune de Mammiferes miocenes de la Grive-Saint-Alban (Isere), etc.," Arch. Mus. d'Hist. Nat. de Lyon, t. v. (Lyon, 1892), pp. 48-50, pi. i. figs. 26-27. |