‘340 MOLLUSCA., 
Mollusea. sufficiently attended to by conchologists, who have, in ge- oy Mollusca. 
A neral, condemned the plan, as founded upon an improper Lavivacag (GENERA: —_—— 
principle, viz., the classification of animals from the places 
which they frequent, instead of the forms which they exhi- 
bit. Such a mode of arranging the higher divisions of the 
different classes we would readily censure; but when em- 
ployed in the inferior subdivisions of the testacea, we regard 
itasan importantandanatural character. We ask the truena- 
turalist to say, which isthe most important character, thehinge 
having teeth or wanting projections, and the animal residing 
in fresh water or in thesea? Weanticipate with confidence 
the preference which would be given to the latter, although 
the decision might provoke a sneer in a mere collector. 
Nature has evidently drawn a line of separation between 
the three tribes, which it is not difficult to perceive. 
The terrestrial testacea are destined to live on vegetable 
matter. Their organs of respiration are suited to the me- 
dium in which they reside. ‘Their organs of feeling are, in 
general, more numerous than those of the fluviatile or marine 
shells. The tentacula of the latter seldom exceed two, 
while in the land shells the tentacula are, in general, four in 
number. The eyes are likewise differently placed; in the 
aquatic testacea they are situate on the head, at the base 
of the tentacula; whereas the eyes in nearly all the terres- 
trial species are placed on the tip of these organs. We might 
also add, that no bivalve shells are found on the land; they 
belong exclusively to fresh water and to the sea. 
The fluviatile shells, though destined to reside in a differ- 
ent medium from the terrestrial, have in the greater number of 
genera their organs of respiration (according to Cuvier) near- 
ly the same, and are, therefore, compelled to come occasion- 
ally to the surface to respire. They have usually two flat ten- 
tacula, with theeyesplacedatthe base. They may, in general, 
be distinguished from the marine kinds by the superior thick- 
ness of their epidermes, their corneous colour, and semi-trans- 
parency. 
The marine shells are the most numerous, the most beau- 
tiful, and the most highly prized of all the testacea. Many 
of the univalves of this tribe possess a lengthened respira- 
tory tube, with a canal in the shell for its protection, a cir- 
cumstance not observed in the fluviatile testacea. There is 
one circumstance which at once points out the difference in 
structure between the fluviatile and marine testacea: the 
-fluviatile cannot live in salt water, nor the marine in fresh 
water. ‘This fact points out an arrangement in their orga- 
nization to which conchologists ought to pay attention. 
These remarks are calculated to persuade conchologists 
to attend to the character furnished by the habitation of 
shells. In the formation of genera, it ought to be respected; 
in the higher divisions it would be inconvenient. The care- 
lessness of Linnzeus with regard to this character, is the prin- 
cipal reason why his genus helix is such a confused.and in- 
digested mass. Were the distinction arising from habitation 
to be observed in the distribution of the testacea, no confu- 
sion could possibly take place. Some changes might be oc- 
casioned by it, but much practical difficulty would be avoided. 
Indeed, so useful is the distinction, that conchologists, with- 
out avowing the propriety of the principle, have in many 
instances observed it. 
Having thus taken a short view of the different characters 
employed by conchologists in the arrangement of the tes- 
tacea, and endeavoured to ascertain their relative import- 
ance, we shall conclude this part of the article by an appli- 
cation of the principles we have established, to an examina- 
tion of the Linnean genera, and to an enumeration of those 
genera which subsequent naturalists have formed, without, 
however, attempting to give even an approximation to many 
-ofthe modern divisions, which have been multiplied beyond 
all bounds ; as our present object is to convey to the reader 
= some farther remarks, illustrative of the history of the science, 
vzather than specific descriptions. 
]. Curroy.—The only change which has taken place in 
this genus, of any consequence, is its transference to the 
naked cephalous mollusca, effected by Lamark. The inha- 
bitants bear a near resemblance to those of the genus patella, 
and belong to the order cyclobranchia of Cuvier. The mar- 
ginal ligament which connects the testaceous plates, even 
after the extraction of the animal, is, in fact, the margin of 
its cloak, and offers, in connection with the notches of the 
plates, more certain and convenient distinctions for the dis- 
tribution of the species, than the number or appearance of 
the valves, a character exclusively employed by Linnzus. 
2, Leras.—This genus has undergone several important 
alterations since the days of Linneus. As originally con- 
structed by that author, it contained shells which differ 
widely from one another in habit and form. Bruguiére, the 
celebrated French conchologist, separated the fixed shells 
furnished with an operculum, under the name of Balanus, 
and those which were seated on a peduncle, he retained un- 
der the generic name Anatifer. He thussuppressed entirely 
the Linnean name of the genus. To the name of his first 
genus, we have no objections, but the second, though it re- 
cords a curious fact in the history of popular errors, has been 
injudiciously selected. The name Lepas has been retained 
py the best British writers, who haye described seven species 
which live in our seas. These are distributed into two sec- 
tions, according as the valves are five or more in number. 
The Lepas anatifera is an example of the first division, and 
the Z. scalpellum of the second. 
The genus balanus, as thus formed by Bruguiére, and re- 
presented by the lepas balanus of Linnzus, contained nine- 
teen species. From these Lamark has separated the B. 
diadema, testudinaris, and balanaris, to form his genus Co- 
ronula. These shells are conical, and have the compart- 
ments formed into twelve areze, six of these being depressed, 
andsix elevated. They chiefly inhabit the skin of the whale, 
the base of the shell being placed in the fat. Lamark has 
likewise formed another genus, from two species analogous 
to the coronolule, which he terms Tubécinella, and charac- 
terises it thus: Testa univalvis, regularis, non spiralis, 
tubulosa, versus basin attenuata, utrique truncata; apertura 
orbiculata terminali; operculo quadrivalvi.” The lepas stri- 
ata of Pennant is now the representation of a new genus 
termed Creusia. : 
M. Dufresne (Annales du Museum, vol.i. p- 465), endea- 
vours to prove by very inconclusive reasoning, that these shells 
are formed posterior to the birth of the animal. He supposes 
that, when they become too small to contain the inhabitants on 
account of their increasing size, the oldshells are forsaken, and 
more commodious dwellings formed, until the animal reaches 
its full size. Other proof, however, than what the author ad- 
duces, is necessary to render the opinion probable. 
Lamark, in his Systeme des Animaux sans vertebres, 
placed these shells in a separate section at the end of the 
bivalves, and among the acephalous mollusca. Afterwards 
he considered them as constituting a particular division of 
the crustaceous animals; and, lastly, he has assigned them 
a place in his new class, which he terms CrrRHIPIDES. 
3. Puowas.—This very natural genus was placed among 
the multivalves by Linnzus, in the twelfth edition of his 
system. It is now united with the bivalves, the accessory 
plates at the hinge being regarded as of subordinate import- 
ance. In other respects it has stood the test of modern in- 
novation, with the exception of the genus GASTROCHENA 
of Spengler, in which the teeth are obsolete. This includes 
the pholas hians of Chemnitz, and the mya dubia of Pen- 
nant. 
4. Mya.—If we consider as definite the character as- 
signed to this genus by Linnzus himself, we shall find that 
-it excludes many species which differ from the M. truncata, 
