ZAP0DIDJ3— ZAPUS HUDSONIUS— CRANIAL CHARACTERS. 471 



as the rostrum at base ; and there is no trace of a postorbital process. The 

 parietals are nearly square, though somewhat emarginate in front, to corre- 

 spond with the convexity of the frontal. There is little, if any, dipping-down 

 of a postero-cxterior angle, so well exhibited in Mus. The interparietal is of 

 large size transversely, though narrow in the other direction ; it reaches across 

 the whole width of the combined parietals, bounding them both posteriorly, 

 ;is it is itself bounded by the occipital. The extent of this narrowly ellipti- 

 cal transverse interparietal is greater than in Mus ; its corner is at a point 

 where the back outer angle of the parietal, back upper angle of the squamo- 

 sal, and front upper angle of the occipital all come nearly together. The 

 squamosal closely resembles that of Mies in size, shape, and connections; 

 there are the same extensive vacuities about the petrosal, with a similar strong 

 clasp, bridging over the opening just above the meatus, running from the root 

 of the zygomatic process to the back edge of the bone. The mastoid is of 

 moderate size, developing nothing to be fairly called a process, wedged 

 between the paroccipital process and the squamosal, at the postero-lateral 

 corner of the skull. It is confluent with the petrosal, but partially fissured 

 away from the surrounding occipital elements. The supraoccipital is of large 

 size and convex contour ; the occipital crest is slight, so that the plane of the 

 occiput is not well defined from that of the superior surface of the skull, the 

 two meeting with a continuous curve, more convex than in Mus. The upper 

 border of the occipital is nearly straight, and bounded quite across by the 

 interparietal ; next comes a considerable piece of squamosal suture, and then 

 the mastoid. The foramen magnum is of great size and nearly hexagonal 

 shape ; most of it being in the plane of the occiput, with only a slight nick 

 interiorly. The condyles are protuberant and convergent ; the condyloid fora- 

 men is close beneath their articular surfaces. The paroccipitals are well- 

 marked perpendicular processes. The basioccipital narrows very rapidly, owing 

 to the strong inward trend of the petrosals, and ends by transverse suture, as 

 usual, with the basisphenoid, opposite the ends of the petrosals. Its under 

 surface shows a pair of slight depressions, with a median ridge. 



The posterior nares are of ample dimensions, owing to the wide separa- 

 tion of the pterygoids. These bones are long, straight, and styloid, with a 

 slightly -clubbed extremity in close approximation to the ends of the petrosals. 

 The palate ends behind with a broad, rounded emargination opposite the last 

 molars. This formation is very different from that of Mus, in which the bony 



