MOLLUSCA. 365 
arms are fleshy in their substance, conical, elongated and Mollusca. 
Mollusca. with a proboscis. Near the middle of the body the shell is 
compressed in their form, and ornamented on the external == 
a —/ attached. The surface of the body above is closely covered 
with small tubercles. It is probable that the species here 
aliuded to is the same with the Péerotrachea coronata of 
Foraskeel. 
Fissurellade. Shell with a slit or perforation. In the 
Fissurella, the apex of the shell is perforated, and united to 
the cloak by a circular muscle open in front. The cloak 
forms a duplicature in front for the branchial cavity, which 
extends to the perforated apex of the shell. The gills con- 
sist of two ridges ; at the dorsal extremity of which is the 
anus. It is probable that the excrements are ejected at the 
perforation in the apex of the shell, and likewise the water 
which enters the branchial cavityin front. The head is fur- 
nished with two tentacula, bearing the eyes at the external 
base. The Patella greca and apertura may be quoted as 
British examples of the genus. 
The genus Emarginula differs from the former in the apex 
of the shell not being perforated. Its place, however, is 
supplied by a slit on the anterior margin, which is the en- 
trance to the branchie and anus. The foot is surrounded 
with a row of filaments, and the eyes are supported on short 
footstalks, characters in which it approaches the genus Hal- 
yotis. The Patella fissura of conchologists is considered 
as the type of the genus. 
Divisron I]—Moxuusca ACEPHALA. 
Destitute of a distinct head, or neck. 
The animals of this group are much more simple in 
their organization than those of the preceding division. In 
none of the species are there any rudiments of organs of 
hearing or of sight. They are destitute of jaws or other 
hard parts about the mouth. They all inhabit the water, 
and possess branchiz. The organs of the two sexes are in- 
corporated in the same individual, and reciprocal union is 
unnecessary. They are either oviparous, or ovoviviparous. 
The presence or absence of a shelly covering, furnish cha- 
racters for a twofold distribution of the groups. 
Sect. 1—Acephala Conchifera. 
The shell in all cases is external and bivalve; and ex- 
hibits very remarkable differences in the form, relative size, 
and connection of the valves. The cloak is likewise in the 
form of two leaves, corresponding with the valves which 
protect it. 
Orver I.—BracnHroropa. 
Mouth with a spiral arm on each side fringed with fila- 
ments. 
The genera included in this group constitute the Bra- 
chiopoda of Cuvier. The lobes of the cloak are free an- 
teriorly. From the body, between the lobes, the arms have 
their origin at the margin of the mouth. These arms are 
capable of folding up spirally. All the species are perma- 
nently attached to foreign bodies, and inhabit the sea. 
Their neryous and reproductive systems have received but 
little elucidation. 
1st Subdivision. 
Shell supported on a fleshy peduncle. 
Genus Lincura. Valves equal, the apex of both at- 
tached to the peduncle. 
The peduncle is nearly cylindrical, cartilaginous, and 
covered with a membrane consisting of circular fibres. The 
valves are oval, flat, and destitute of teeth, or elastic liga- 
ments. The adductor muscles are numerous, obliquely 
placed, and appear capable of giving to the valves a con- 
siderable degree of lateral motion. The cloak is thin, and 
has interspersed muscular fibres. Its margin is thickened, 
and fringed with fine hairs of nearly equal length. The 
surface with thick set fringes or tentacula. The mouth is 
simple, and situate between the arms at their base. There 
is no enlargement of the alimentary canal, which can be re- 
garded as a stomach, and the anus isa simple aperture situ- 
ate on the side. There are marked indications of salivary 
glands and a liver. The blood is conveyed to the gills by 
two vessels, which are divided at the separation of the lobes 
into two branches, one of these going to the half of one 
lobe, and another to the opposite half of the other lobe. 
Two systemic veins occupy a similar position, and return 
the aérated blood to the two lateral systemic ventricles. 
The gills themselves are arranged in a pectinated form, on 
the inner surface of each lobe of the cloak. There is no- 
thing known of the nervous or reproductive systems of this 
animal. 
The Lingula unguis is the only species of the genus, and 
appears to be confined to the Indian seas. The valves were 
first figured by Seba, together with the peduncle by which 
they are supported. lLinnzeus having seen only one valve, 
conjectured that it belonged to Patella, and named it P. 
unguis. Chemmitz examined both valves, without the pe- 
duncle, and pronounced them connected with the genus 
Pinna. Bugiere, aware of Seba’s figure, contemplated the 
formation of the new genus for its reception, which Lamark 
executed. M. Cuvier afterwards dissected one of the indi- 
viduals, which Seba had possessed, and unfolded characters 
in its organization, sufficient not only to warrant the con- 
struction of a new genus, but a new class. 
Some petrifzctions have recently been referred to this 
genus ; but, in the absence of all vestige of the peduncle, 
we do not consider the mere form of the sheil as furnishing 
characters sufficiently obvious and precise to warrant such 
distribution. 
TEREBRATULA. Valves unequal, the peduncle passing 
through an aperture in the largest valve. 
The following interesting information, from the dissec- 
tions of Mr. Owen, will be read with pleasure by the student 
of the anatomy and physiology of the mollusca. 
The mantle adheres very closely to the valves: the lobe 
which corresponds to the perforated valve is traversed longi- 
tudinally by four large vessels; the opposite lobe is simi- 
larly traversed by two such vessels. Its margins are thick- 
ened, not as in the Lamillibranchiate Bivalves from con- 
traction, but owing to a peculiar structure connected with 
respiration. They are puckered at regular distances, the 
puckerings being apparently caused by the insertion of de- 
licate ezlia, which pass as far within the mantle as they pro- 
ject out of it, but which are so minute as to be observable 
only by means of a lens. In the interspaces of the cilia the 
margin of the mantle is minutely fringed, and within the 
fringe is a canal, which extends along the whole circumier- 
ence. From this canal the large vessels of the mantle lobes 
take their origin: they may be regarded as the branchial 
veins conveying the aerated blood to the two hearis, which 
are situated exterior to the liver, and just within the origin 
of the internal calcareous loop: they are accompanied in 
their course by much smaller vessels, probably the branchial 
arteries. Such is apparently the system of respiration in 
Terebratula. 
The viscera occupy a very small space near the hinge. 
The alimentary canal commences by a small puckered 
mouth, situated immediately between the folded extremi- 
ties of the arms. It passes backwards, and expands into a 
membranous stomach, surrounded by the liver, a bulky gland. 
of a green colour and minute follicular texture, which com- 
municates with it by many orifices. The intestine passes 
down to the hinge, and then turns to the right side and 
terminates between the two mantle-lobes. No trace of a 
salivary gland was found. 
