NATURAL SCIENCES OP PHILADELPHIA. 311 



and forming the greater part of the roof of the brain-case. Thus 

 we have the curious circumstances of extensive mastoideo-occi- 

 pital suture on top of the skull, and still more prolonged mas- 

 toideo-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 occi- 

 pital bones respectively is distinct 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 pe- 

 trosal 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 distin- 

 guished 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 mark corre- 

 sponds to a nearly complete bony wall within the bone, which 

 partitions off one sinus from another. This may be hereafter 

 found to indicate the respective parts which certain primitive otic 

 elements take in the formation of the "mastoid." 



The petrosal, i. e., the bulla ossea, is less peculiar than the mas- 

 toid ; it is not more inflated than in very many mammals, and is 

 chiefly remarkable for its contact with its fellow, and for the ex- 

 tensive uninterrupted fissures which separate it both from basi- 

 occipital and alisphenoid. In general shape it is conoidal, mode- 

 rately swollen, with the apex of the cone produced and curiously 

 curved toward the median line of the skull, where it meets its 

 fellow, forming a bony bridge beneath the basisphenoid. The 

 claw of the hamular pterygoid rests against the end of the petro- 

 sal ; and close to this there is an abutment of a piece of the sphe- 

 noid ; otherwise, there is a great fissure betwixt it and the sphe- 

 noid. It is only in contact with occipital elements by means of 

 the flange-like exoccipitals ; the whole extent of the basioccipital 

 being separated, as just said, by a large fissure. Posteriorly it is 

 confluent with the mastoid, with imperfect indication of the pre- 

 cise line of union ; exteriorly it is continuous, without appreci- 

 able 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 Geomyidse 



