~ 
SENSE ORGANS OF AMIURUS. 381 
ascending process with the claustrum, but the elaustrum can hardly 
be said to have any relation tothe atriwm. 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’, 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 AM/URUS, 
When exposed in situ is found to be covered by peritoneum which is 
reflected on to the cesophagus 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 
uls-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 Hasse—loc cit, 591. 
