SACCOMYID2Z—DIPODOMYIN AX—DIPODOMYS. 527 
mark corresponds to a nearly complete bony wall within the bone, which par- 
titions off one sinus from another. This may be hereafter found to indicate 
the respective parts which certain primitive otic elements take in the forma- 
tion of the “mastoid”. 
The petrosal, 2. ¢., the bulla ossea, is less peculiar than the mastoid; it 
is not more inflated than in very many mammals, and is chiefly remarkable 
for its contact with its fellow, and for the extensive uninterrupted: fissures 
which separate it both from basioccipital and alisphenoid. In general shape 
it is conoidal, moderately swollen, with the apex of the cone produced, and 
curiously curved toward the median line of the skull, where it meets its fel- 
low, forming a bony bridge beneath the basisphenoid. The claw of the 
hamular pterygoid rests against the end of the petrosal; and close to this 
there is an abutment of a piece of the sphenoid; otherwise, there is a great 
fissure betwixt it and the sphenoid. It is only in contact with occipital ele- 
ments by means of the flange-like exoccipitals; the whole extent of the basi- 
occipital being separated, as just said, by a large fissure. Posteriorly it is 
confluent with the mastoid, with imperfect indication of the precise line of 
union; exteriorly it is continuous, without appreciable indication of original 
distinction with the special inflation in which the meatus is situated. This 
papery vestibule I presume to be analogous with the tubular meatus externus 
in Geomyide and elsewhere; the orifice: is large, subcircular, and simply a 
hole without raised brim, pierced in the back upper corner of the bulb. Ante 
riorly the tympanic bulges so far as to form part of the orbit. While this 
special inflation is not otherwise distinguished from the general bulla ossea 
than by a slight constriction, it is remarkably divided off, above and behind, 
from the mastoid, by a strong line of impression, of which I shall say more 
presently. 
’ 
Coincidently with the hypertrophy of these otic elements of the tempo- 
ral, the squamosal is peculiarly reduced in extent, and pushed into the orbit, 
to which it is almost entirely restricted. It is simply a small irregularly 
shaped plate of bone lining the back part of the orbit, with a slight spur just 
exceeding the orbital brim in a little notch between corners of the frontal 
and parietal bones. The squamosal remains long discrete from all its sur- 
roundings. In full-grown though youngish animals, the squamo-sphenoidal 
sutures may be readily traced—that with the alisphenoid just below the 
-glenoid cavity, that with orbito-sphenoid within the orbit. The zygomatic 
