SENSE ORGANS OF AMIURUS. 381 



ascending process with the claustrum, but the claustrum can hardly 

 be said to have any relation to the atrium. It lies dorsally to it, but 

 its thin edge bears no such relation to the roof of the atrium as the 

 claustrum does in Cyprinus 1 , nor can it have any influence on the 

 shape of the atrial cavity. The patch is further connected with the 

 thickened cushion of dura mater which partly closes the apertura ex- 

 terna atrii. The cushion is somewhat horseshoe-shaped, the convex- 

 ity fitting into the notch of the exoccipital before referred to, while 

 •the ligament of the stapes fills up the concavity. Of the two arms 

 the lower is connected with the patch of dura mater, the upper be- 

 hind the claustrum with the oblique-stripe referred to above, which 

 possibly represents the ascending process of the incus. 



THE AIR-BLADDER OF AMIURUS, 

 When exposed in situ is found to be covered by peritoneum which is 

 reflected on to the oesophagus by the air-duct. Outwardly it appears 

 to be oval in form and undivided. It is formed of a thick tunica 

 externa and a delicate tunica interna which contains very few vessels. 

 If the external tunic be cut into, the internal tunic may be removed 

 readily without its collapsing. It differs at first sight from the outer 

 in form, for its anterior third is impair, while its posterior end is 

 formed of two separate sacs opening into the anterior one. A nearer 

 examination of the external tunic shows that it is also divided pos- 

 teriorly by a median vertical partition forming two chambers in 

 which the sacs of the internal tunic are received. Immediately in 

 front of the ventral end of the partition is the orifice of the air- 

 duct which thus opens into the anterior chamber. The partition 

 does not terminate by a sharp edge, but splits as it were into two 

 flattened bands which are attached dorsally to the vertebral column, 

 and slant downwards and forwards as they grow wider to become 

 continuous with the ventral surface of the air-bladder. They narrow 

 the apertures of communication between the posterior and anterior 

 parts of the air-bladder, and simultaneously form two small ventral 

 culs-de-sac from the posterior chamber on either side of the median 

 partition. Except for these bands the posterior part of the bladder 

 is entirely free from the vertebral column ; it is only in the anterior 

 division that we have to look for certain connections with the osseous 



1 Hasw—loc cit, 591. 



