AIR-BLADDER IN XOTOPTERFS BOR>'EKJfSIS. 513 



IV. EEIiA-TIOHS or THE AlR-BLADDER TO THE SkULL AND 



Auditory Organs. 



The structure of the tubular prolongation of the subspherical 

 sac and of the auditory caeca, and the relations of both to the 

 skull and the auditory organ, will be better understood after a 

 brief description of certain structural features in connection 

 with the hinder part of the skull (PL 37. fig. 4). 



The basioccipital (b.o.) is somewhat hourglass-shaped, being 

 slightly constricted at the junction of the hinder third with the 

 anterior two-thirds of its length, and expanded towards either 

 extremity, but much more so in front than behind. The hinder 

 half of the bone is produced ventrally into a laterally-compressed 

 bony keel, which is traversed by a moderately deep, median, longi- 

 tudinal groove, bounded by prominent lateral ridges, and trans- 

 mitting the initial section of the dorsal aorta- 

 Traversing the lateral surface of the basioccipital, immediately 

 ventrad to the sutural articulation of this bone with the ex- 

 occipital and opisthotic, is a well-marked oblique ridge, which, 

 on each side, extends backward and downward and finally 

 terminates in a free projecting process {a.p.\ closely applied to 

 the ventro-lateral surface of the centrum of the first vertebra 

 after the fashion of an accessory articular process. Between 

 the aortic groove ventrally and the oblique ridge dorsally, the 

 lateral surface of the basioccipital is traversed by a fairly deep 

 groove (b.ff.), the direction of which is obliquely upward and 

 forward towards the outer surface of the auditory capsule of 

 the same side. Anteriorly to the commencement of the aortic 

 groove the basioccipital ceases to be laterally compressed, and, 

 instead, becomes greatly swollen laterally, assuming in fact a 

 distinctly bullate appearance. The lateral surface of this portion 

 of the bone is also traversed by a well-defined groove, which, 

 however, is merely an extension forwards of that meationei 

 above. Ventrally, the anterior half of the basioccipital is in 

 relation with the hinder portion of the parasphenoid (ps.), and in 

 front the bone articulates by means of an irregular squamous 

 suture with the hinder margin of the prootic (pro.). 



There is a complete series of periotic bones, which, in the 

 main, exhibit the usual relations one to another and to the 

 adjacent cranial bones. The opisthotic (op.) forms the postero- 

 lateral portion of the auditory capsule, articulating with the 



