RODENTIA SCIURINAE. 241 



no perforation in this plate at all, the infra-orbital foramen for the passage of the nerve opening 

 far forward, except in Tamias, some distance beyond the. molars, nearly in a line with them or 

 just above and close to the process of the maxillary, near the palate for the attachment of the 

 tendon. The foramen is oval, as in Tamias, or triangular, (Spermophilus,) or vertically com 

 pressed, (Sciurus,) and is situated a short distance anterior to the molars, with a bony margin, 

 generally tubercular, below. 



The malar bone is of great extent, as in the family generally. Anteriorly, it extends for 

 ward, and is wedged between the lachrymal and the maxillary ; posteriorly it reaches behind 

 further than the process of the temporal, and constitutes the outer, and in part the posterior 

 boundary of the glenoid cavity. 



The descending ramus of the lower jaw is nearly quadrate, its upper posterior angle acute 

 and directed upwards from the line of the condyle ; and the lower posterior angle rounded and 

 directed inwards. The lines formed by the lower margin of the descending ramus on each side 

 are nearly parallel. The horizontal rami meet in front and join by a symphysis of limited 

 extent. 



The molars are normally ^, (sometimes, in the young? |^). The anterior upper molar is, 

 however, deciduous at an early age in many American squirrels, (especially the fox-colored 

 ones.) reducing the formula to ^. The anterior upper molar, (where there are five,) is much 

 smaller than the rest, with a simpler crown and a single root. The other molars are simple, 

 with three or four roots. Their crowns are rhomboidal or sub-trigonal ; the grinding surface 

 with four transverse ridges going nearly across, one on the anterior and posterior margins, 

 and two intermediate, usually most elevated ; these are sub-parallel or sub-convergent. 



The body is covered with hair varying from extreme softness to great coarseness. The eyes 

 are well developed. Ears are variable from a mere rudiment to a considerable length. Tail 

 sometimes longer than the body, sometimes only one-tenth the length ; always densely coated 

 with hair, usually longest on the sides. 



The coecum is large, the clavicles perfect, the tibia and fibula distinct, and the upper lip cleft. 



In an excellent monograph of the Spermophiles, Brandt 1 distinguishes the squirrels proper 

 from the marmot squirrels by certain characters which may serve to systematize and simplify 

 the consideration of so large a group as that of the Sciurinae, as given above. To the squirrels 

 proper he applies the term Campsiurina, which includes Sciurus, Xerus, Pteromys, and Tamias, 

 while the Arclomyina embrace Arctomys and Spermophilus, with its sub-genera, Coldbotis and 

 Otospermophilus. The characters of these tribes are as follows : 



Tribe I. CAMPSIURINA. True squirrels. First upper molar much the narrowest, oblong- 

 linear, very simple, shorter than the second, very rarely almost equal in length and about one- 

 eighth the size, conical or sub-conical, very often deciduous. Crowns of the greater molars, 

 except the last, sub-tetragonal or rhomboidal, a little narrowed internally, with two central 

 folds or ridges, nearly parallel, and sometimes with one or more accessory ridges. The anterior 

 dentiform tubercles of the lower molars exceed but slightly, or not at all, in height the height 

 of the crowns. The ridges of the soft palate between the molars are equal in number or nearly 

 so to the molars, quite distinct. The fourth finger often, the third not rarely, longer than the 

 rest. Claws trigonal at the base, strongly compressed and much hooked, generally shorter than 

 the fingers. 



1 Sur les differentes espSces des Souliks de Russie, &c., par J. F. Brandt. Bulletin Physico-Mathematique de 1'Acad. Imp. 

 des Sciences de St. Petersbourg, II, 1844, 358. 



31 L 



