526 MONOGRAPITS OF NORTH AMERICAN RODENTIA. 



the development of the temporal bone, 1 can only describe it as it appears in 

 the adull 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" tlie 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, Geomyidce. 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 trabeculse 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 

 ll.it tened 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 



