~MOLLUSCA 
Mollusca. Clio and Limacina, frequent the arctic regions, and afford 
‘~~ the whale a great part of its sustenance. None of the spe- 
355 
dermon Peroni, the trivial name being in honour of the Mollusca. 
discoverer, M. Peron. Saya 
cies have hitherto been detected in the British seas. 
M. Cuvier divides the animals of this class into eephalous 
and acephalous. In the latter division he places the genus 
hyalea. The head of the animal of this genus, with its in- 
ferior neck, may, however, be sufficiently recognised to re- 
move all doubt of its existence. 
The characters which may be employed in the classifica- 
tion of this group are numerous; but the influence which 
their different forms exercise on the habits of the species is 
still unknown. The following disposition of the genera, 
though it has no claims to a natural division, may be useful 
to the student in his investigations. 
(1.) Tunie Strengthened by a Shell. 
Genus Lmracuya.—Posterior extremity of the body co- 
vered by a spiral shell. 
The shell, which is very tender, makes one turn and a 
half, is flat on one side, with a large pillar cavity on the 
other. The fins*are two in number, one on each side of the 
neck. When the animal swims, the head with the fins are 
protruded. 
This genus was instituted by Cuvier, for the reception of 
the Clio helicina of Captain Phipps, or Argonauta arctica 
of Fabricius. According to Mr. Scoresby, it is found in 
great quantities near the coast of Spitzbergen. 
Genus Hyarea.—Posterior extremity of the body pro- 
tected by two connected shelly valves. 
In the animals of this genus, the body is lodged between 
two plates or valves, united at the base, where they inclose 
the caudal extremity. The ventral valve is nearly flat, with 
an uneven margin, narrow anteriorly, but expanding behind, 
and terminating in three projecting points. From the mid- 
dle point four ribs diverge forward, and a muscle arises, 
which, fixed in the superior viscera, enables the animal to 
withdraw into the shell. The dorsal valve is shorter than 
the preceding, the margin flat and circular, and the middle 
convex outwardly. The branchiz are situate in the space 
between the lateral margin of the two valves, on ‘each side, 
in a duplicature of the tunic, the sides of which are furnished 
with filaments. The fleshy neck supports the two mem- 
branaceous expansions; between which and the base the 
mouth is situate, surrounded by two lips, and strengthened 
within by two fleshy cheeks. The opening of the anus and 
oviduct are at the base of the right fin. 
The Hyalea tridentata, the best known species of the 
genus, was first noticed by Forskal, in his Descriptiones Ani- 
malium, p. 124, as an Anomia, and inhabiting the Mediter- 
ranean. The same species was likewise taken in abundance 
in the Gulf of Guinea, by the expedition under Captain 
Tuckey. 
(2.) Tunie destitute of a Shell. 
A. Fins double. 
Posterior extremity with leaf-like ridges. 
Genus PrEumMopERMOoN—Head with twobundles of ten- 
tacula. 
The body is oval, with a narrow neck, and a fin on each 
side. The mouth is nearly terminal, furnished on each side 
with a fleshy lip, and beneath, with a fleshy chin. Each 
tentaculum consists of a filament, with a tubercle at the end, 
pierced by a small hole, and considered as exercising the 
office of a sucker. Cuvier, in his Mémoire sur ? Hyale et 
le Pneumoderme, considered the leaf-like ridges which oc- 
cur on the caudal extremity of the body, as the branchiz, 
and even describes the pulmonary vein which conveys the 
blood from these to the heart. But, in his Réegne Animal, 
he states it as the opinion of M. Blainville, that the fin-like 
expansions of the neck contain the branchiz on their sur- 
face, as in the case of Clio. The rectum and oviduct ter- 
minate under the right wing. Cuvier has figured and de- 
scribed the only known species, which he terms Pnewmo- 
Posterior extremity simple. 
Genus Crr0.—Body ovate, with the tunic elongated and 
membranaceous. 
The head is divided into two lobes, the summits of which 
are furnished with tentacula. The existence of eyes has 
not been ascertained. The mouth is transverse, with two 
lateral longitudinal lips. On each side of the neck arise two 
blunt, conical, fin-like expansions, with a fine reticulated 
surface, considered as serving the double purpose of fins 
and branchize. The anus and orifice of generation termi- 
nate under the base of the right branchia. .The viscera do 
not fill entirely the cavity of the inner bag. The gut makes 
only one fold. 
The genus Clio was originally instituted by Brown in 
his Natural History of Jamaica. It was afterwards em- 
braced and modified by Linnzus and Pallas, in such a man- 
ner as ultimately to exclude the species for the reception of 
which Brown originally formed it. It contains two species, 
the most remarkable of which is the Clio borealis. Mr. 
Scoresby, in his valuable work on the Arctic Regions, states, 
(vol. i. p. 544), that it occurs in vast numbers in some situ- 
ations near Spitzbergen, but is not found generally through- 
out the arctic seas. In swimming, it brings the tips of the 
fins almost into contact, first on one side and then on the 
other. 
Genus CrroporA.—Body covered with a triangular py- 
ramidal tunic. 
The fins are membranaceous. The mouth is situate be- 
tween these, and is furnished with a semicircular lip. This 
genus was instituted by Peron, for the reception of the Clio 
of Brown. The C. pyramidata is the best ascertained spe- 
cies. Brown’s Jamaica, p. 386, tab. 43, £1... Two other 
species were taken by the Congo expedition, in S. lat. 2. 14., 
and E. long. 9. 55., and 5S. lat. 2.41., E.long. 9. 16., “both 
having a spinous process on each side of their shell, near its 
opening. ‘One species is beautifully sulcated transversely, 
and the other but slightly so.”—Tuckey’s Narrative, p.412. 
B. Fin single. 
Genus CymBuiia.—Tunic cartilaginous and trough- 
shaped. 
The fin is single, divided into three lobes, one of which 
is small, with two tubercles, and a minute fleshy beard. 
This genus was instituted by Peron, in Annales du Museum, 
t. xv. t. 3, f. 10, 11. : 
Sect. I.—Gastrropopa.— Organs of Progressive Mo- 
tion fitted for Creeping. 
This is one of the most extensive groups of molluscous 
animals. The marks by which it is distinguished are well 
defined, and the external and internal characters of the spe- 
cies have been successfully illustrated. 
The Gasteropoda may be considered as having the body 
protected dorsally by the cloak, and ventrally by the foot. 
The cloak is either continuous, and usually more or less 
arched, for the reception of the viscera underneath, or it is 
interrupted by a projecting bag, in which are contained the 
principal digestive and reproductive organs. This project- 
ing bag is tapering and spiral, and always protected exter- 
nally by a shell. When the cloak is continuous, the surface 
is variously marked, and frequently exhibits a particular 
portion more elevated than the rest, in some cases conceal- 
ing a testaceous plate, which has been termed the shiedd. 
The foot situate on the ventral aspect, and in opposition 
to the cloak, exhibits a flat, soft surface, consisting of inter- 
laced muscular fibres. Its central surface serves as a sup- 
port to the viscera, while externally it constitutes the organ 
of progressive motion. It is a sucker rather than a foot, 
and enables’ the animal to adhere to objects when at rest, 
and to crawl from one place to another by a succession of 
