192 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
between the siphons. At this point of concrescence of the 
two ctenidia, a peculiar little semicircular flap of mem- 
branous tissue projects downwards and forwards into the 
general mantle cavity. 
This horizontal shelf is further prolonged anteriorly by 
the inner lamelle of the internal branchiew. Part of these 
inner lamelle (B7r.J.2, figs. 8 and 4) have the ordinary 
filamentar structure and are connected to the outer lamellee 
of the same branchie by the inter-lamellar junctions. 
But the remainder of the inner lamelle are simply mem- 
branous, and fuse with each other across the middle line 
of the body, and thus form the floor of the ventral supra- 
branchial chamber (M.c.4, fig. 4), which continues forward 
the cavity of the dorsal siphon. The- outer reflected 
lamellee of the external branchize, on the other hand, 
fuse with the body-wall just beneath the origin of the 
mantle lobes. It is evident from a consideration of fig. 4 
that the inner and outer lamellee of each external branchia 
must enclose a cavity, which is also continuous with the 
cavity of the dorsal siphon; this is the dorsal suprabran- 
chial chamber (M.c.5, fig. 4). The ventral suprabranchial 
chamber is single and median. ‘There are paired, right 
and left, dorsal suprabranchial chambers. 
Since the inner lamelle of the internal branchiz do not 
fuse with the body-wall but with each other, the ventral 
suprabranchial: chamber is incomplete anteriorly ; the dorsal 
chambers end blindly in front. Evidently water entering 
the general mantle cavity by the ventral siphon or through 
the ventral opening of the shell has three courses open to 
it. It may pass forward between the labial palps into the 
mouth and so reach the dorsal siphon per anwm ; it may 
enter the ventral suprabranchial chamber through the 
space included between the posterior surface of the foot 
and the anterior margin of the fused inner lamellg of the 
