AIU-BLA.DDEB, IN NOTOPTERUS BOBNEEKSTS. 515 



the lumen of the bony cul-de-sac and the perilympli-containing 

 cavity of the auditory capsule. 



Returning now to the air-bl'addei' (PI. 38. fig. 3), it may be 

 stated that the dorsal surface of the tubular prolongation (t.p.) 

 of the subspherical sac is closely moulded to the yentral surface 

 of the centrum of the first vertebra, and also to the ventral and 

 lateral surfaces of the hinder part of the basioccipital. From 

 its laterally-compressed shape, the ventral portion of the latter 

 bone forms a median keel projecting dov^nwards and pushing 

 before it the dorsal wall of the subjacent part of the air-bladder, 

 which, consequently, appears as if partially subdivided internally 

 by an incomplete, but very thick, longitudinal partition (1-2^.), 

 while dorso-laterally the bladder fills up the grooves on the 

 lateral surfaces of the basioccipital. Laterally and ventrally, 

 this portion of the air-bladder is free from any special relations 

 or attachments to the skeleton, and hence its moderately thick 

 walls consist of both tunica externa and tunica interna : elsewhere, 

 however, the walls are intimately related to the skeleton, and 

 then are either firmly attached thereto or become greatly reduced 

 in thickness. Thus, dorso-laterally, on each side, the tunica 

 externa ceases by becoming inserted into the oblique bony ridge 

 on the lateral surface of the basioccipital. In the mid-dorsal 

 line, the tunica externa is not only considerably thickened by 

 mesial fibres derived from the arch -like band which forms the 

 free ventral margin of the longitudinal septum of the sub- 

 spherical sac, but is also attached to the lips of the bony aortic 

 groove in such a way as to convert the groove into a canal. 

 Between the mid-dorsal and the dorso-lateral skeletal attach- 

 ments, however, the tunica externa is wanting, and all that 

 represents the proper wall of this part of the bladder is the thin 

 tunica interna investing and lining the hinder portion of the 

 grooves on the lateral surfaces of the basioccipital, into which 

 the lateral portions of the bladder are received. The diameter 

 of the tubular portion of the bladder is about 6 mm. 



The two auditory caeca (l.a.c, r.a.c.) communicate with the 

 preceding part of the bladder by relatively wide orifices ; and at 

 this point the simple non-septate lumen of each caecum is about 

 3'5 mm. in diameter. From their origins the two caeca diverge 

 obliquely upward and forward towards the outer surfaces of 

 their respective auditory capsules. In its forward extension 

 each auditory caecum traverses the anterior portion of the oblique 



