PTEROPODA. 
21 
three points. The mouth with two small tentacula is situated be- 
tween the wing's towards the closed side of the shell and above two 
small eyes, and the genital aperture, whence issues a small penis in 
the shape of a little proboscis. It is so diaphanous, that the heart, 
brain, and viscera can be distinguished through the envelopes*. 
Pneumodermon, Cuv. 
The Pneumoderma begin to be a little further removed , from the 
Clios. Their body is oval, without a mantle and without a shell ; the 
branchiae are attached to the siu'face, and composed of little laminae, 
arranged in two or three lines so disposed as to form an H on the 
part opposite to the head The fins are small; the mouth which is 
furnished with two small lips and two bundles of niimerous tentacula, 
each terminated by a sucker, has a little lobe or fleshy tantaculum 
beneathf. 
P neumodermon Peronii, Cuv. Ann. du Mus., IV, pi. 59 ; 
and Peron, Ib., XV, pi. 2. Not more than an inch long. The 
species known was captured in the Ocean by Peron. 
Limacina^ Cuv. 
The Limacinae, according to the description of Fabricius, should have 
been closely related to the Pneumoderma ; but their body terminates 
in a spirally convoluted tail, and is lodged in a very thin shell formed 
by one whorl and a half, unbilicated on one side, and flattened on the 
other. The animal uses its shell as a boat, and its wings as oars, 
whenever it wishes to navigate the surface of the deep. 
The species kiiOAvn Clio helicina, Phips and Gmel. -^Argonauta 
arctica, Fab., Faun. Groenl., 387, is almost as common on the 
Arctic seas as the Clio borealis, and is considered as forming 
one of the chief sources of food for the Whale*. 
PIyalea, Lam,^ — Cavolixa, Ahildg. 
Have two large wings ; no tentacula ; a mantle cleft on the sides, 
lodging the branchiee in the bottom of its fissures, and invested by a 
shell also cleft laterally, the ventral face of which is arched, and the 
dorsal flat and longer than the other ; the transverse line which 
unites them behind, is furnished with three sharp dentations. When 
alive, the animal thrusts several appendages, that arc more or less 
* See Peron, Ann. Mus., XY, pi. iii, f. 10 — 11. N. B. in the fig. of Ci/mbvUa, 
given by Blainville, Malac., XLYI, the position of the animal in the shell is directly 
the reverse of the true one. Our description is founded upon the recent and re- 
peated observations o€ M. Laurillard. 
f AI. de Blainville-once thought that the fins supported the branchial tissue, and 
that what I have considered as branchisc is another kind of fin. In this case the 
analogy with the Clios would have been greater ; hut since then, (Alalacol., p. 483) 
that gentleman has adopted my views. 
X I am not sure that the animal drawn by Scoresby, of which dc Blainville 
(Malac., pi. xlviii. bis, f. 5) makes his genus Spiratella, is, as he thinks, the same 
as those of Phips and Fabricius. 
