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Show 66 MR. E. R. ALSTON ON THE ORDER GLIRES. [Jan. 18, The difference between the mandible characteristic of the Sciuro-morpha and My o morph a and that peculiar to the Hystricomorpha will be best shown by a comparison of the figures*. In the more typical forms the infraorbital opening is not enlarged to give passage to a portion of the masseter muscle ; and in all the malar extends far forward, and is not supported below by a continuation backwards ot the maxillary zygomatic process. The incisive foramina are small, and confined to the intermaxillaries ; the foramina of the base of the skull are proportionally small; and there is no interpterygoid canalt*. The clavicles are always perfect, the posterior ridge of the scapula is strongly developed, and the acromion is broad and flattened. Externally the muffle is naked, the upper lip usually cleft, the nostrils rounded above and comma-shaped, the ears hairy, and the tail cylindrical and well haired, except in Castor, in which it is flattened and scaly. _ The typical family, the Sciuridce, easily distinguished by their postorbital frontal processes, has been divided for convenience into two subfamilies, the long-tailed arboreal Squirrels (Sciurinee), and the short-tailed terrestrial Marmots (Arctomyince), though it must be confessed that their differences are merely adaptive and not very striking. The other families are all more or less aberrant, and their true affinities have been the subject of much discussion. The first of these is the Anomaluridce; and I have already*}; given my reasons for considering that it must be regarded as an undoubted though specially differentiated family of this section. The sciurine affinities of the Haplodontidce, in spite of its peculiar dental and cranial characters, have been definitely established by Dr. Peters§, although Prof. Lilljeborg has strangely relegated it to the Hystricomorpha^. The position of the remaining family, Castoridee, has been a still more vexed question, ever since the Beaver has been extricated from the old jumble with the Musquash and the Coypu. Professor Gervais appears to have been the first, to treat Castor as an aberrant member of the present group %, in which Mr. Water house** and Professor Bairdft have concurred ; and although these writers have not been generally followed, it seems evident to m e that we must revert to their views. Professor Brandt fully recognized that in all the more important points the osteology of Castor agrees with that of the Sciuromorpha, but considers this resemblance to be negatived by the external habitus and manner of life, as well as by the structure of the teeth, feet, and tailj'f. Prof. Lilljeborg places the * By permission of Professor Flower the illustrations have been drawn from specimens in tbe Museum of the Royal College of Surgeons. t This name was proposed by Mr. Waterhouse for the fissure which in some rodents leads from the bottom of the pterygoid fossa into the orbit. Cf. Turner, P. Z. S. 1848, p. 63. + " O n Anomalurus, its Structure and Position," P. Z. S. 1875, pp. 88-97. § Monatsb. Ak. Berlin, 1864, p. 177. || Op. cit. p. 9. 1 Diet. Univ. d'Hist. Nat. xi. p. 203. ** Physical Atlas, Zool. map, 5 (letter-press). tt North-American Mammals, p. 350. \\ Op. cit. pp. 149, 150. |
