MOLLUSCA. 133 
sions. They have their origin on the ventral side of the 
mouth, between that organ and the funnel. The suckers 
are pedunculated, with their margin strengthened by a corn- 
eous ring, furnished with teeth. The sac is furnished with 
_ fin-like expansions, and strengthened internally by corneous 
or testaceous ribs or plates. ‘The head is divided from the 
sac on all sides by a neck. The margin of the anus is sur- 
rounded with tentacula. 
Genus Serra. The sac is furnished on each side through- 
out its whole length with a narrow fin. 
The suckers are irregularly scattered on the arms and 
feet. The back is strengthened by a complicated calca- 
reous plate, lodged in a peculiar cavity. This plate has 
been long known in the shop of the apothecary under the 
name Cuttle-fish bone, which was formerly much prized in 
medicine as an absorbent, but is now chiefly sought after 
for the purpose of polishing the softer metals. It is some- 
what ovate, flatly convex on both sides, and thickest where 
broadest. The superior haif, or the one next the head, is 
the longest, rounded at the extremity, and thin. The in- 
ferior portion becomes suddenly narrow, and ends in a point. 
It may be considered as consisting of a dermal plate, con- 
cave on the central aspect, having its concavity filled up 
with layers which are convex on their central aspect. 
According to our observations, the dermal plate appears 
to consist of three different lamine, arranged parallel to one 
another. The external or dorsal layer is rough on the sur- 
face, and marked by obscure, concentric arches towards 
the summit, formed by minute knobs, which become larger 
towards the base, where they appear in the form of inter- 
rupted transverse ridges. It is uniform in its structure, and 
the tubercles possess a polish and hardness equal to porcel- 
