MOLLUSCA. 359 
ed with bloodvessels. The duct proceeds from this mass, Mollusea. 
Mollusca. triradiated apertures. It is uncertain whether the animal 
undergoes for a short space a sudden thickening of its walls, <\—=’ 
= —e is to be considered as dextral or sinistral. This genus was 
instituted by us several years ago, for the reception of the 
Nautilus lacustris of Lightfoot, first described and figured in 
Phil. Trans. vol. txxvi. p. 160. tab. 1. f. 1, 8. 
Genus Prronra.—The body is destitute of the external 
protection ofa shell. The head is furnished with two long 
retractile tentacula. The snout is divided into two broad ap- 
pendages. Between the tentacula, towards the right side, is 
the opening forthe penis. The anusis terminal, immediately 
above which is the entrance to the pulmonary cavity; and on 
the right is the opening to the female organs, from which a 
groove runs towards the right lobe of the snout. The mouth 
is destitute of a proboscis or jaws. The tongue is merely 
a cartilaginous plate grooved transversely. The gullet is 
long in proportion, with a villous surface. There are three 
stomachs, each distinguished by its peculiar characters. 
The first is a true gizzard, covered internally with a carti- 
laginous cuticle, and its walls formed of two strong muscles, 
with connecting ligaments. The second stomach is funnel- 
shaped, with prominent ridges both onits external andinternal 
surface. These ridges, at their origin internally, are high- 
est, and project considerably into the cavity, acting like a 
valve in retarding the progress of the food. The third 
stomach is short and cylindrical, covered internally with 
equal longitudinal fine ridges. The intestine is nearly of 
equal thickness throughout, and upwards of'twice the length 
of the body. The salivary glands are much branched, and 
pour their contents into the entrance of the gullet. The 
liver, in the animals of this genus, is distributed into three 
separate portions, each of which may be regarded as a dis- 
tinct liver, an arrangement which is not known to take place 
in any other animal. ‘The first liver is situate near the mid- 
dle of the body, on the right side; while the second is 
placed near the posterior extremity. The ducts enter the 
cardiac opening of the stomach, each by a separate aperture, 
and seem to occupy the place of the zone of gastric glands 
observed in birds. The third liver is placed at the posteri- 
or end of the gizzard, into which it pours its contents by a 
short duct. 
The most remarkable feature of the circulating system, 
is the position of the lungs at the posterior extremity of the 
body, which occasions a corresponding arrangement in the 
connecting organs. The entrance to the pulmonary cavity 
is immediately above the anus.. The vessels in which the 
blood is aérated, are distributed on- the roof and sides of 
the cavity. The pulmonic veins consist of two receptacles, 
one on each side, extending nearly the length of the body, 
which may considered as venz cave. These receive the 
blood by numerous vessels, and convey it directly to the 
lungs. The aérated blood is conveyed by a systemic vein 
into a large auricle, seated in front of the lungs, of consider- 
able size, with the walls fortified on the interior by branch- 
ed ligaments. The ventricle is placed at its anterior ex- 
tremity, and separated by two valves. ‘The aorta arises 
from the opposite side of the ventricle, its main trunk pass- 
ing on towards the head. 
The male and feniale organs of generation, although oc- 
curring in the same individual, appear to occupy different 
parts of the body. The opening of the male organs is at 
the tentacula, which leads to a cavity terminating in two 
unequal recesses. The anterior is the smallest, and receives 
the termination of a vessel three or four times longer than 
the body, which takes its rise at the external base of the 
cavity, apparently from the cellular substance, and, after a 
variety of convolutions in the neighbourhood of the mouth, 
opens into the recess. The second recess is-the largest, and 
the vessel connected with it is most complicated. Its ori- 
gin is in amass which occupies a considerable portion of the 
abdominal cavity, and which consists-of a vessel forming a 
great number of complicated convolutions; liberally suppli- 
after which it again contracts, and, before it terminates in a 
perforated glandular knob in the recess, it contains a pe- 
dunculated fleshy body, with a sharp-pointed corneous ex- 
tremity, probably capable of being protruded into the recess 
and cavity. The parts which are considered as forming the 
female organs, or those which are connected with the sexu- 
al cavity on the right side of the anus, consist of an ovari- 
um, divided into two lobes, each of which may be perceived 
to be again minutely subdivided. The oviduct is tortuous, 
and passes through a glandular body, which, in the other 
gasteropoda, is regarded as the testicle. The pedunculated 
vesicle gives out two ducts, one of which goes to the testi- 
cle, the other to the uterus. It is difficult to form even a 
conjecture regarding the uses of all this complicated sexual 
apparatus. The subject can only be elucidated by an at- 
tentive examination of the condition of the organs at differ- 
ent seasons of the year, and by studying, at the same time, 
the habits of the animals. 
The preceding description of tle characters of the genus 
is taken from the anatomical details of a species found 
creeping upon the rocks under water in the Mauritius, by 
M. Peron, which Cuvier referred to the genus Onchidium 
of Buchanan, already noticed. We have ventured to insti- 
tute the genus, and to name it in honour of the discoverer 
of the first ascertained species. Cuvier conjectures that it 
breathes free air, and has accordingly inserted it ameng the 
pulmones aquatique. Some doubts, however, may reason- 
ably be entertained as to the truth of this supposition. It 
would certainly be an unexpected occurrence, to find a mar- 
ine gasteropodous mollusca obliged to come to the surface, 
at intervals, to respire. It will probably be found that it is 
truly branchiferous. 
Cuass I].—Brancuirera. 
The molluscous animals of this class are more numerous 
than those of the preceding. They chiefly inhabit the wa- 
ters of the ocean, a few genera only being met with in fresh 
water lakes and rivers. The branchiz which constitute their 
aérating organs, exhibit numerous varieties of form, posi- 
tion, and protection, and furnish valuable characters for their 
methodical distribution. 
Orper I.—Brancul® EXTERNAL. 
The branchize are pedunculated, and more or less. plu- 
mose. They are moveable at the will of the animal, and, 
im general, are capable of great alteration of form. 
Grnus Doris.—Oral tentacula two; vent without scales. 
The cloak is covered with retractile papilla, and separated 
from the foot by a distinct duplicature. Towards its ante- 
rior margin are placed the two superior tentacula. ‘These 
are retractile, surrounded at the base with a short sheath, 
and supported on a slender stem, having an enlarged com- 
pound plicated summit. The neck is short, and above the 
mouth there is a small projecting membrane, connected at 
each side with the oral tentacula, which are in general mi- 
nute, and of difficult detection. The mouth is in the form 
of a short trunk, leading to fleshy lips, within which the 
tongue is placed. This last organ is covered with minute 
reflected hairs, and, from its motion, appears to be destined 
exclusively for deglutition. The gullet is a simple mem- 
branaceous tube, terminating in a stomach, which presents on 
the interior a few longitudinal folds. It is furnished with a 
small cecum, the extremity of which receives the bile from 
the liver. The stomach likewise receives the secretion of 
another gland, which is not connected with the liver, in the 
form of a small bag, the inner surface of which is covered 
with numerous papillee. The intestine is lodged in a groove 
on the surface of the liver, and proceeds directly to the 
anus. The liver itself is divided into two lobes, and gives 
