COVERED-GILLED MOLLUSKS. 129 
GENUS PLEUROBRANCHUS. 
A not unapt comparison has been made of these 
Mollusks to a tortoise, the mantle representing the 
back shield, and the foot the breastplate. They are 
more or less oval in outline, flattened, with the 
back convex; the mantle is fleshy, and projects 
considerably on all sides ; the head is placed beneath 
its front margin, and carries two superior tentacles 
with a pair of minute eyes, sunk beneath the skin 
at the outer part of their bases. The upper part of 
the head is developed into a ‘7 
broad triangular veil, the lateral “iff (iy 
edges of which carry another (/, 7 
pair of tentacles. These organs, 
as well as the superior pair, are 
imperfectly tubular, and are 
formed by the bending together 
of their sides, which meet and & 
overlap, but do not unite. The s 
mouth is a short, fleshy pro- \s 
boscis, provided with horny jaws ‘& 
and a long ribbon-like tongue, 
armed with teeth, as in the Nu- 
dibranchs. The branchial plume 
projects from between the mantle 
and foot; in crawling it 1s trans- 
verse, and appears, when viewed 
from above, to be composed of Ab 
many triangular lamine, over- Sth PLUME OF FLEURO- 
lapping and pointing backwards. 
Each lamina shows transverse wrinkles. Viewed 
laterally, it is seen to consist of a central stem, 
with about eighteen pinne on each side, each pinna 
being again pinnated on each side The stem, 
