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1 



52G 



MONOOUAPnS OF NORTH AMEIUCAN KODENTIA. 



I ! 



llifi (lovrlopmmit <»f Hid temporal l)oiio, T ciin only drsrribo it ns it nppcnrH in 

 the udult uiiiiiml, imiiff tlio toims "mastoid", "petrosal", and "tympniiic" in 

 thoir current acceptation, without reference to tlie primitive otic elements. 

 Respecting the scpiamosal, there is no difficulty; the petrosal a)>pears under 

 its usual condition of a bulla ossea. I regard as "tympanic" the inflated vesi- 

 cle in wiiich the meatus auditorius is pierced, which appears as a tui)ular 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 Hie skull. Excepting the reduced squamosal, all the elements arc sub- 

 jected to extraordinary inflation, as well as peripheral enlargement; they 

 appear as papery vesicles, light, thin, and siuooth, 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 trabecule within, and imperfectly partitioned into several 

 cavities by thin septa. The mastoid constitutes the greater part of tiicsc 

 bulging masses. Its backward protuberance occupies nearly oil 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, 

 tilling 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 tlie right-angled parietal articulates with the mos'toid; 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 squamosol, 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 jwrtion 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 



