00" MR. E. R. ALSTON ON THE ORDER GLIRES. [Jail. 18, 



Tlie difference between the mandible characteristic of the Sciuro- 

 rnorpha and Myomorpha 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 of 

 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 canal?. 

 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 Seiuridce, easily distinguished by their 

 postorbital frontal processes, has been divided for convenience into 

 two subfamilies, the long-tailed arboreal Squirrels (Sciurina), 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 Jnomaluridce ; 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 Haplodontidcb, 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 Hys- 

 tricomorpha^. The position of the remaining family, Castoridce, 

 has been a still move 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 ^f, in which Mr. Water 

 house** and Professor Bairdyj have concurred ; and although these 

 writers have not been generally followed, it seems evident to me 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 tailJJ. Prof. Lilljeborg places the 



* By permission of Professor Flower the illustrations have been drawn from 

 specimens in the 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 bottoiu of the pterygoid fossa into the orbit. Cf. Turner. 

 P. Z. S. 1848, p. 03. 



I " On Anomalurus, its Structure and Position," P. Z. S. 1875, pp. 88-97. 

 § Monatsb. Ak. Berlin, 1804, p. 177. || Op. tit. p. 9. 



1[ Diet. Univ. d'Hist. Nat. xi. p. 203. 



** Physical Atlas, Zool. map, 5 (letter-press). 



tt North-American Mammals, p. 350. 



I I Op. tit. pp. 1 49, 1 50. 



