92 OBSERVATIONS ON THE RODENTIA. 



many other points indicative of the mutual affinity of those 

 placed in the same section. 



Two genera, however, form exceptions, not possessing the 

 combination of characters above noticed ; I allude to Dipus 

 and Helamys. The whole of the skulls were therefore re-ex- 

 amined, with a hope that the discovery of some other cha- 

 racters might enable me to determine the situation of these 

 genera. I then perceived that the lower jaw afforded points 

 of distinction of great importance, by means of which the af- 

 finities of one of the two genera just mentioned [Dipus), can, 

 I think, be satisfactorily determined. 



In order the more clearly to describe the lower jaw, I shall 

 view each ramus as divided into four portions, which may be 

 called the alveolar portion, the condyloid, the coronoid, 

 and the descending ramus. — A ridge of bone on the inner 

 side of the ramus, extending from the alveolar portion to the 

 condyle,forms the upper or anterior boundary of what I term 

 the descending ramus. 



In the first section, which I shall call Murina, the de- 

 scending ramus of the jaw consists of a broad plate, concave 

 on the inner side, and flat or convex on the outer. It ap- 

 proaches more or less to a quadrate form, the upper posterior 

 angle is directed outwards, and the lower posterior angle, 

 which is often rounded, is directed inwards. The lower boun- 

 dary of this plate consists of a thickened ridge or branch, 

 which springs from the under side of the alveolar portion of 

 the jaw, and is directed backwards and downwards. The 

 posterior part of the descending ramus is usually in the same 

 perpendicular line as that of the condyle, and very seldom 

 extends beyond that line. 



The coronoid process terminates usually high above the 

 level of the molars. The condyloid portion is long and di- 

 rected obliquely upwards and backwards. The rami of the 

 jaws converge to a point at the symphysis, which is usually 

 of but small extent, and forms an angle of about 45° with the 

 horizontal ramus. 



The lower jaw of a squirrel may be regarded as the type 

 of the form just described. I shall have occasion to notice 

 the departures from this type when I define the families. 



The principal genera contained in this section are Sciurus, Arctomys, 

 Myoxus, Dipus, Mus, Arvicola, Geomys, and Castor. 



Section II. — Hystricina. In this section the descending 

 ramus of the jaw is formed by a triangular flattened plate, the 

 lower boundary of which consists of a thickened ridge or 

 branch, (the under surface of which is almost always flat) 

 which springs from the outer side of the alveolar poition, and 



