SACCOMYID^— DIPODOMTINiE— DIPODOMYS. 529 



subcircular, and mostly in the perpendicular plane of the bone. After in- 

 closing this orifice, the bone rapidly contracts as it rises to the top of the 

 skull ; this part is marked with a sharp perpendicular median ridge, and the 

 edges of the bone being affected in coming into apposition with the swelling 

 mastoids, there results a pair of deep narrow fossae upon the face of the bone. 

 Tlie portion of the supraoccipital which mounts the top of the skull to there 

 lie horizontal, almost immediately forks to embrace a small shield-shaped 

 interparietal bone between its prongs. The ends of these prongs touch 

 posterior corners of the parietals. 



The sphenoid bone is of rather small extent, owing to the situation of 

 the squamosals in the orbit. It is widely fissured from the petrosals. The 

 alisphenoid is very short; its termination may be seen in the jagged suture 

 with the squamosal ; but short as it is, it only misses taking part in the 

 mandibular articulation, since it extends to the margin of the glenoid. The 

 orbito-sphenoid is comparatively smaller still, the place it occupies in Geo- 

 mijidez for instance being here largely occupied by the squamosal. The 

 spheno-palatal suture may be traced in young specimens with a zigzag but 

 still in general transverse course, from the side a little behind the maxillary 

 alveolus across the middle line of the skull. There is no vertical orbital 

 plate of the palatal bone ; it all lies flat, and extends forward on the palate, 

 wedge-shaped, but with square termination to a point opposite the first true 

 molar. A backward spur of this bone forms with its fellow a sharp median 

 process. There are various foramina already noted. The pterygoids are 

 small claw-hammers abutting at their extremity against the petrosals. 



The parietals are nearly right-angled triangles, with one side of mutual 

 apposition along the median line of the skull, another transversely articulating 

 with the frontal, and the hypothenuse postero-exterior, for the mastoid suture. 

 The back corners meet the prongs of the occipital and slightly embrace the 

 interparietal. The outer corner is prolonged into a spur which attains the 

 brim of the orbit. And here, the remarkable construction of the orbit by an 

 unusual number of bones, may be noted. Following the brim of the orbit 

 around we find — zygomatic process of maxillary ; lachrymal; frontal; spur 

 of parietal ; back upper corner of squamosal ; front end of mastoid ; fore 

 bulge of tympanic ; zygomatic heel of squamosal ; whole of malar, and so 

 back to maxillary. 



The portion of the frontal which appears on the surface of the skull is 

 24 M 



