SACCOMYID.E— DirODOMTIN^]— DirODOMTS. 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, i. e., 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 GeomyidcB 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 



