(the " masseteric plate " described above), wbicb forms a great 

 flying buttress for the support of the fore-part of the arch ; the 

 superior maxillary root, forming the roof of the infraorbital canal, 

 is very short. The distal end of each maxillary zygomatic process 

 is expanded into a broad fork which receives the anterior end and 

 much of the lower border of the jugal; this bone is always very 

 short and is confined to the central portion of the arch, bridging 

 the small interval between the maxillary and squamosal zygo- 

 matic processes. The squamosal root of the zygoma is relatively 

 weak. 



The nasals are relatively short, often ending anteriorly well 

 behind the front faces of the incisors, and never conspicuously in 

 advance of them ; posteriorly they terminate usually either a little 

 in front of or on a level with the anterior margin of the orbit. 



The interorbital region is always clearly defined, but in its 

 degree of constriction it differs considerably in different genera and 

 species, as well as in different stages of growth in one and the same 

 individual, tending as a rule to become narrower with age. In 

 many forms the temporal ridges, which always extend far forwards, 

 approach each other and in adults fuse to form a median crest 

 in the interorbital region; in other forms the ridges though 

 salient remain more or less widely separated by a median sulcus ; 

 in many genera the ridges are feeble and both the interorbital 

 region and the interval between the ridges in that region remain 

 wide and flat even in old age. 



On the roof of the braincase the temporal ridges usually 

 diverge from the hinder part of the interorbital region to points 

 above and nearly in the same transverse plane as the glenoid 

 articulations ; thence the ridges converge ■ slowly to the hinder 

 part of the parietal region and then again curve outwards, passing 

 the extremities of the interparietal to join the front face of the 

 occipital crest. Throughout their course, after leaving the 

 frontals, these ridges tend to follow the upper edge of each 

 squamosal, but they impinge upon the parietals anteriorly and 

 cross the lateral wing of each parietal in the post-glenoid region. 

 In some genera (Ondatra, Prometheomys, and Ellohius) the 

 temporal ridges are closely approximated throughout in the later 

 stages of growth; and in Prometheomys they form a salient 

 sagittal crest which, in old age, extends from the interorbital 

 region to the occiput. 



The squamosals are always largely developed, forming a great 

 deal of the sides of the braincase. In this subfamily no post- 

 orbital processes are ever formed by the frontals ; but very charac- 

 teristic post-orbital crests are developed upon the squamosals for 

 the attachment of a tendinous portion of the anterior part of the 

 temporal muscle on each side; in some genera these crests are 

 represented by prominent peg-like processes. In genera in which 

 the temporal ridges form an interorbital crest, the squamosals 

 frequently show a tendency to approach each other anteriorly, 



