MOLLUSCA. 343 
to unite such as are irregular in their form and imbricated Mollusca- 
Mollusca. it a new genus, which he calls Aippopus. In its hinge it t 
in their structure, with such as are of regular growth and === 
~~~ resembles the tridacna, but differs in the structure of the 
lunule, which in.this is closed. 
The genus Drceras of Lamark, which he formed from 
the chama dicornis of Bruguiére, approaches the isocardia 
in appearance, but the following character which he assigns, 
is fully sufficient for their discrimination: “ Testa bivalvis 
ineequivalvis, adherens: natibus conicis, maximis, divergen- 
tibus, in spiram irregularem contortis. Dens cardinalis 
maximus, crassus, concavus, auricularis in valvula majore. 
Impressiones duo musculares.” It occurs only in a fossil 
State. 
Before dismissing this Linnzean division of shells, we must 
notice another genus which has beenadded to it by Lamark, 
from species brought from the Indian Seas. He terms it 
etheria, and originally described its generic character in the 
following words : “ Coquille bivalve, inequivalve, irreguliére, 
adherente, a crochets court, enfonces dans la base des 
valves et deriges de cote. Charniére sans dent ; deux im- 
pressions musculaires separées et laterales. Ligament demi- 
interieur, enveloppant une callosite oblongée, et sortant en 
dehors par une fissure recourbée.” He has described four 
species which are very rugged on the outside, but finely 
nacred within, and has placed the genus in his family ca- 
macea, while in external aspect, and in the ‘absence of 
teeth, the species make a near approach to the ostreacea. 
13. Arca. Linneus assimilated, under this genus, every 
shell the hinge of which presented numerous mutually inserted 
teeth. The shells which were thus united, have numerous re- 
lations, and constituteavery natural family. But in this family 
there are several groups of which Berguiére formed sections 
and Lamark genera. The genus arca is now restricted to 
those shells in which the hinge is in a straight line, and 
composed of numerous small lamelliform teeth, without la- 
teral ribs. They have obtained their name from their re- 
semblance to a ship, when the shell is inverted. Many 
species of this genus gapé a little at the superior margin, to 
enable the animal to send out those tendinous threads by 
which it adheres to the rocks. The A. lactea, noe, tetra- 
gona, barbata, and fusca, are natives of the British Seas. 
The Linnzan arc, whigh have the hinge line broken 
and angular, belong to the genus Nucura. In this genus 
the beaks are contiguous and turned a little backwards. The 
nucula, nuclea, minuta, rostrata, and tenuis, are found on 
our shores. 
In the genus Pectuncutvs, the hinge teeth are situated 
on a curved line, the shell is nearly orbicular, and the mus- 
cular impressions, which are two in number, form each a 
callous projection with a sharp margin. The arca pilosa, a 
native of Britain, is referable to this genus. 
In the genus CucuLima, the teeth of the hinge are simi- 
lar to the arca, but at each extremity there are three or 
four transverse parallel ribs. It is represented by the arca 
cucullata of Chemn. (Conch. vii. p.174. tab. 55. f. 526-528.) 
To the family arcacea, Lamark, at one period, added 
the genus which he terms Trrconra. The hinge teeth are 
only two in number, diverging and compressed, but they 
are transversely grooved on each side. The muscular im- 
pressions are two in number in the recent species, T. pect- 
znuta, but in some of the fossil shells referred to in this ge- 
nus, Mr. Sowerby could observe only one. The trigonia 
now forms along with castalia, the small family of Trigonées 
formed by Lamark at the suggestion of M. Valenciennes. 
14. Ostrea.—Linnzus, in the construction of this ge- 
nus, brought together many shells totally dissimilar in form, 
character, and habit, and hence it has undergone great al- 
terations in the hands of succeeding conchologists. To as- 
sociate in one genus shells which remain immoveably fixed 
to the rocks and stones from their birth, and which exhibit 
few other signs of vitality than the opening and shutting of 
their valves, with those which possess a locomotive power ; 
solid texture, might surely be regarded as a violation of all 
the laws of a natural or an artificial system. Yet of such 
incongruous materials is the Linnean genus ostrea compos- 
ed, which, in spite of all its imperfections, has still its ad- 
mirers in this country. (See Descriptive Catalogue, Lin. 
Trans.) The first important improvement in the reforma- 
tion of the genus, consisted in the separation of the pec- 
tens, which was executed by Pennant, and afterwards by 
Bruguiére and Lamark. Since new characters have been 
assigned to the genus ostrea, other separations must take 
place. It is thus defined: “ Testa bivalvis inzequivalvis, 
rudis adherens ; cardine edentulo. Fossula cardinalis ma- 
joris valve ztate crescens. Ligamentum semi-internum. 
Impressio muscularis unica.” The genus may be divided 
into two sections; the first having the margin of the valves 
simple, as the common oyster; and in the second the mar- 
gins are plaited, as in the O. crista-galli. 
In consequence of this change in the generic character, 
the ostrea malleus of Linnzeus, (Lister, tab. 219, f. 54.,) has 
been formed into the genus Mateus. The shell is free, 
gapes a little at the beaks, produces a byssus, has no teeth 
in the hinge but a conical pit for the insertion of the liga- 
ment, placed obliquely on the margin of each valve. It 
was for a long time highly prized by collectors. 
The genus PECTEN is one of thebest cha:acterised, most 
natural, and most beautiful, in the system. The shell is 
inequivalve and regular, the hinge is destitute of teeth, and 
the internal ligament is fixed to a triangular cardinal cavity. 
There are fourteen species natives of our shores. 
From the ostrea perna, ephippium, and isogonum, Bru- 
guiére formed the genus Perna. The hinge is linear, and 
cut into a number of lengthened parallel veins, which re- 
ceive the ligament. The interstices are formed into teeth, 
which simply oppose those of the other valve. In the an- 
terior side of the valve, near the beaks, there is a callosity, 
and an opening for the byssus of the animal. 
Lamark has constituted another genus, nearly allied to 
the pernz, which he styles CRenatruLa. The hinge in this 
genus presents only a row of pits for the ligament, which 
makes it appear crenulated. ‘The intermediate spaces are 
not formed into teeth, neither is there any callosity, or 
opening for the byssus. He has figured two new species, 
which he terms avicularis and mytiloides, and a third is the 
ostrea picta of Gmelin, (Chemn. Conch. vii. p. 243. tab. 38. 
f. 575.) 
Nearly related to the pectens is the genus Lima of Bru- 
guiére. The species differ, however, in the ligament being 
in a great measure on the outside. They are all of a white 
colour. The ostrea dima is considered as the type of the 
genus. 
The genus Pepum of Bruguiére differs from the preced- 
ing in the ligament being external, and attached toa lon 
straight fissure. The ostrea spondiloidea of Chemn. (Conch. 
viii. t. 72. fig. 669, 670,) is considered as the type of the 
genus. 
To this family we must add two genera, possessed of very 
singular characters. They have neither hinge nor ligament. 
The first, instituted by the celebrated botanist, Commer- 
son, is termed Acarpo. The valves are depressed and 
nearly equal, and held together by the adductor muscle. 
The species at present known come from the eastern coast 
of Africa. The second is termed radzolites, and was insti- 
tuted by Lamark. It differs from the former in the form 
of the valves, the inferior being turbinated, and the supe- 
rior convex or conical. The species oceur only in a fossil 
state, and have been long known to geognosts under the 
title ostracites. 
15. Anom1a. In the Linnzan system, this genus is equ- 
ally faulty as the last. It contains many species, Which 
