SACCOMYID4—DIPODOMYIN.B—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 fossee upon the face of the bone. 
The 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 extents 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- 
myide for instance being here largely ovcupied 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 oe appears on the surface of the skull is 
34M 
