OBSERVATIONS ON THE RODENTIA. 95 



which is produced downwards into an angle more or less dis- 

 tinctly marked. Zygomatic process of the maxillary bone 

 consisting of a broad thin plate, which is concave in front, 

 oblique in its position, (the lower part being the farthest re- 

 moved from the nasal portion of the skull), and occupies the 

 whole space between the plane of the palate, and that of the 

 upper surface of the skull. The lower boundary of this plate 

 is emarginated, and forms an arch which throws the anterior 

 portion of the zygomatic arch above the plane of the palate. 

 Palatine portions of the intermaxillary, maxillary, and pala- 

 tine bones, on the same plane, or together, forming a slightly 

 concave and nearly even surface. Incisive foramina small 

 and narrow, terminating in most cases at the intermaxillary 

 suture. The palatine portion of the palate-bone ap- 

 proaching to a quadrate form, the palato -maxillary suture 

 being almost always between the penultimate molars : there 

 are two small, widely separated, suturo -palatal foramina ; 

 and on each side, close behind the last molar there is a tole- 

 rably large posterior palatal foramen. The posterior bound- 

 ary of the palate is doubly emarginated or truncated, and is 

 situated in a line with the hinder portion, the last molar on 

 each side, or {generally) behind this line. 



Frontal bone with a distinct post-orbital process, which is 

 directed backwards and downwards, and leaves a wide space 

 for the passage of the temporal muscle. The malar bone is 

 continued forwards and upwards, to join the lachrymal, and 

 backwards, to form the outer boundary of the glenoid cavity; 

 this cavity is broad and open, and not contracted by longitu- 

 dinally elevated ridges. J 11 



Lower jaw. — The descending ramus nearly of a quadrate 

 form, its upper posterior angle acute and directed outwards 

 from the line of the condyle, and the lower posterior angle 

 rounded and directed inwards. The lines formed by the low- 

 er margins 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 most striking feature in the skulls of the present 

 family, and one which distinguishes them from all other 

 rodents with which I am acquainted, is the distinct post- 

 orbital process. This process however, although always 

 distinct, varies considerably in size. It is most developed in 

 the larger species of the genus Pteromys. In some of the 

 marmots it is also very large. In the genus Sciurus it varies 

 considerably, but neither in this genus nor in any other of the 

 present family > have I ever found it wanting. It is least de- 

 veloped in the palm squirrel, [Sciurus palmarwm). In some 



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