526 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
the development of the temporal bone, I can only describe it as if appears in 
the adult animal, using the terms “mastoid”, “petrosal”, and “tympanic” in 
their current acceptation, without reference to the primitive otic elements. 
Respecting the squamosal, there is no difficulty; the petrosal appears under 
its usual condition of a bulla ossea. I regard as ‘‘tympanic” the inflated vesi- 
cle in which the meatus auditorius is pierced, which appears as a tubular pro- 
longation of the bulla in the nearest allied family, Geomyide. The rest of 
the inflation, forming the greater part of the occipital surface and much of the 
roof of the cerebral cavity, I shall simply designate as ‘‘ mastoid”. 
The two temporal bones together are little less bulky than all the rest 
of the skull. Excepting the reduced squamosal, all the elements are sub- 
jected to extraordinary inflation, as well as peripheral enlargement; they 
appear as papery vesicles, light, thin, and smooth, without ridges or angles, 
inclosing extensive antra, the collective capacity of which is scarcely less than 
that of the whole brain-case. These vaulted walls are supported by delicate 
bony arches or trabeculze within, and imperfectly partitioned into several 
cavities by thin septa. The mastoid constitutes the greater part of these 
bulging masses. Its backward protuberance occupies nearly all the occipital 
surface of the skull on each side, the occipital bone being reduced to a nar- 
row margin of the foramen magnum, sunk in an emargination between the 
mastoid and its fellow. On the top of the skull, the mastoid similarly expands, 
filling the whole of the area usually occupied by the squamosal, and forming 
the greater part of the roof of the brain-case. Thus we have the curious 
circumstances of extensive mastoideo-occipital suture on top of the skull, and 
still more prolonged mastoideo-parietal suture—for the whole of the longest 
side of the right-angled parietal articulates with the mastoid; while so great 
is the anterior prolongation of this same bone, that a small part of it fairly. 
enters the orbit at the back outer corner of the latter. This extensive line 
of sutures with squamosal, parietal, and occipital bones, respectively, is dis- 
tinct throughout; but the boundaries of the mastoid with other otic elements 
can only be inferred by certain lines of impression which appear to mark it 
off from petrosal and tympanic. Another point is to be considered here: the 
flattened and entirely superior portion of the mastoid (that which lies in the 
ordinary site of a squamosal) is incompletely distinguished from the occipital 
portion of the same bone by a line of impression running straight across from 
the margin of the meatus auditorius to the median line of the skull; and this 
