RODENTIA—SACCOMYINAE—DIPODOMYS. 407 
however, to the anterior edge of the skull. The parietals are nearly right angled triangles 
in shape, bounded behind by the interparietal and the occipital, postero-laterally by the 
temporal, and sending a narrow process round the edge of the orbit of which it forms the pos- 
teror-internal corner. 
The temporal bone is of very great extent and of unusual development—the two, in fact, 
constituting more than one-half of the entire skull. This development is principally in the 
mastoid portion, the sinuses of which are very large. There is no outer tube to the meatus 
externus, which opens directly into the bone; the cavity, however, with a thin wall. As 
already stated, this highly cellular mastoid portion constitutes the most posterior part of the 
occiput ; the two of opposite sides, in fact, in their projection, presenting a ludicrously close 
resemblance to the buttocks of the squatting human figure. The petrous portions, too, are of 
large size, and conical ; they project forwards and inwards until they come in contact below 
the spheroid, a* little anterior to its junction with the base occipital; the union distinctly 
visible externally. The squamous portion lines the posterior half of the orbit. 
The occipital bone is most remarkable in its reduced dimensions; it may be considered in 
three portions: a posterior, inferior, and superior. The posterior portion is merely an inverted 
V, with narrow and linear branches, which, coalescing above into one, pass a short distance 
around on the superior surface of the head, when the bone is again bifurcated ; the branches 
linear, and but little divergent, and extend forward to near a very short posterior process of 
the parietal. The branches thus formed enclose a nearly quadrilateral and elongated inter- 
parietal, the anterior end of which alone rests against the parietal. The inferior portion of the 
occipital, again, is Y-shaped, the fork bounding the lower portion of the foramen magnum, and 
bearing the narrow and elongated condyles, the body of the Y extending forward and abutting 
against the anterior and united ends of the petrous bone. There is a narrow fissure along the 
whole of the basilar and condyloid portion of the occipital, between it and the petrous bone. 
The foramen magnum itself is very large, much higher than wide, and situated two-thirds in 
the posterior portion of the occiput, and one-third in the inferior. 
The maxillary bone extends far forward on the sides, reaching almost to the incisors. Just 
within its lateral suture with the intermaxillary, and about on the middle of the side of the 
snout, is a very large perforation in the bone, opening directly into the nasal cavity. This is 
nearly as large’as the tip of the muzzle, the aperture rounded anteriorly, straight posteriorly. 
This perforation, which I have never seen anywhere else except in Perognathus, seems to 
replace the ante-orbital foramen, of which there are no traces. The zygomatic plate of the 
maxillary is of moderate extent, and un its superior portion it sends backwards a broad plate, 
overhanging the orbit for about one-third, and forming a portion of the upper surface of the 
head. It is united to the frontal, and the lachrymal bone is situated in the angle between 
these two bones. The inferior edge of the zygomatic plate, springing from the side of the 
maxillary, has a very deep rounded notch bounded externally by a narrow acute process, which, 
passing backwards, articulates with the very slender needle-like malar bone. The posterior 
end of the malar abuts directly against the petrous bone, although also articulating with the 
squamous. 
The lower jaw is slender and delicate, in fact, unusually small in proportion to the size of 
the skull. The distance from the molars to the incisors is very short, and there is very little 
contraction towards the incisors, the depth being much the same from the incisor-alveoli to the 
coronoid process. The postero-inferior angle of the jaw is twisted so as to occupy an angle of 
more than 45 degrees with a vertical. plane, the upper angle being produced upwards and 
