310 PROCEEDINGS OF THE ACADEMY OF 



rendering it almost necessary to their stability that they should 

 abut against each other at their extremities ; but there is also a 

 singular separation of the alveolar portion of the maxillaries 

 from the superincumbent parts, by a horizontal fissure ; so that the 

 palate appears as a sort of bridge between the fore and aft parts 

 of the skull. Various peculiarities will appear more clearly from 

 the following descriptions of individual bones. 



The complex temporal "bone" will be first considered, since 

 the prime peculiarities of the skull result from the wholly singular 

 condition of hj'pertrophy under which several of the elements of 

 this bone exist. The topography of the parts, and the connections 

 of the bone are curiously remodelled as a consequence of the 

 enormous inflation of the various otic elements, and corresponding 

 reduction of the squamosal. In the absence of investigations into 

 the development of the temporal bone, I can only describe it as 

 it appears in the adult animal, using the terms " mastoid," "pe- 

 trosal," 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 con- 

 dition of a bulla ossea. I regard as "tympanic" the inflated 

 vesicle in whicli the meatus auditorius is pierced, which appears 

 as a tubular prolongation of the bulla in the nearest allied family, 

 Geomyidse. The rest of the inflation, forming the greater part of 

 the occipital surface and much of the roof of the cerebral cavity, 

 I shall simpty 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 subjected to extraordinary inflation, as well as pe- 

 ripheral 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-cavity. 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 these bulging masses. Its backward protuberance occupies 

 nearly all the occipital surface of the skull on each side, the occi- 

 pital bone being reduced to a narrow 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, 



