528 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
process of the squamosal is of peculiar character; instead of a slender curved 
spur reaching around to grasp the malar, there is a short abrupt heel appressed 
against the tympanic, and to the roughened face of this heel the clubbed end 
of the needle-like malar is affixed. The relation of the parts is such that the 
zygoma appears to articulate behind with the tympanic—it actually has an 
abutment against that bone, though no real articulation with it. 
From the lower back edge of the squamosal, a curious thread of bone 
starts off and occupies the deep groove already mentioned as separating the 
tympanic from the mastoid. No break from the squamosal can be seen in 
this thread, which curls around the orifice of the meatus, still in the groove 
mentioned, and ends by a slightly enlarged extremity below and behind the 
meatus, exactly in the position of an ordinary ‘mastoid process”. I am 
uncertain of the meaning of this. The end of this ligule or girdle of bone 
thus encircling the tympanic is in the site of the postero-lateral angle of the 
skull in Geomyide, in which such angle is formed by a corner of the squamo- 
sal; and the inference is self-suggestive that this delicate bony strap may - 
an edge of the squamosal persisting in situ after the rest 
really be squamosal 
of that bone has been crowded down into the orbit by the encroachment of 
the mastoid. Such a view, however, will bear further scrutiny. Even if a 
slender spur of actual squamosal does run out into the tympano-mastoid 
groove, it does not follow that the whole of the fold in this groove is squamo- 
sal; and certainly the enlarged extremity of this ridge, behind the meatus, 
has every appearance of an ordinary mastoid process. 
Next after the squamosal, the occipital bone suffers most from the 
enlargement of the otic elements; it is singularly restricted in extent, and 
presents itself in unique shape, compressed between the swollen mastoids. 
All the lateral occipital suturation is with the mastoid, excepting the basi- 
occipital. The occipital lies in three planes, nearly at right angles with each 
other. The basioccipital is horizontal, as usual, upon the floor of the skull; 
the exoccipitals, with probably part of the supraoccipital, are perpendicular 
behind; the rest of the supraoccipital is horizontal again, on top of the skull. 
The basioccipital is wedge-shaped, and offers nothing very peculiar, except- 
ing its entire disconnection from the petrosals, between which it lies; its 
sphenoidal articulation is just behind the joined apices of the petrosals. 
Exoccipitals appear as a pair of flaring flange-like processes, just outside the 
condyles, appressed against the otic capsules. The foramen is very large, 
