MOLLUSCA, 
the dextral species may form the genus Spirorbis, while Mollusca. 
“Mollusea. there is always an opening like a key-hole, near the summit 
=~ Of the shell. 
The &. greca and apertura are found on our 
coasts. 
The genus Emarerivta is readily distinguished by the 
slit or indentation which occurs on the posterior margin of 
the shell. 
In the genus Caruxts of Montfort, the shell is conical, 
with the summit produced into a beak, more or less recurv- 
ed, and twisted. The P. hungarica of Lister is the type. 
The genus Concuotepas is furnished with an opercu- 
‘lum, and in form and habits approaches the buccinum. It 
is represented by the P. integra of Da Costa’s El. tab. 2. 
fig. 7. 
“In the genus Crepipuna, the cavity of the shell is par- 
tially interrupted by a simple diaphragm. The P. poreed- 
lana is the representative of this genus. The C. chinen- 
"sts inhabits the British seas. ' 
In the preceding genus, the first approach to the turbi- 
nated shell makes its appearance, which becomes more ob- 
vious in the genus CALyptTrR2A, in which the cavity is fur- 
nished with a spiral diaphragm. The C. equestris is the 
type of the genus, which is related in part to the trochi. 
From this genus of Lamark, Montfort has separated the In- 
FUNDIBULUM, as possessing a central spiral pillar. Sow- 
erby has figured several species of this last genus in his 
Mineral Conchology as occurring in a fossil state in Bri- 
tain. 
The patella unguis now ranks as a bivalve, and consti- 
tutes the genus Liyeuxa in the acephalous family brachi- 
opoda of Lamark. Linnzeus, who never saw more than one 
valve, placed it among the patella. Chemnitz, who ex- 
amined both valves, considered it as a pinna. These writ- 
ers had overlooked the figure of the perfect shell, with its 
tube or stalk, as given by Seba, vol. iii. fig. 16. No. 4. This 
specimen, which belonged to Seba, passed into the museum 
of the Stadtholder, and afterwards reached, in company with 
the spoils of the other Continental collections, the museum 
of Paris. Here Lamark examined it, and formed his new 
genus. And the same specimen enabled Cuvier to inves- 
tigate its anatomical structure, which he has explained in 
detail in the first volume of the Annales de Museum. Science, 
in this instance, as well as several others, profited by the 
successes of the late emperor of the French. This genus 
is destitute of a hinge. The valves are supported on a pe- 
duncle, and the shell is opened partly by the relaxation of 
the adductor muscle of the animal (and not by the external 
membrane, as stated by Mr. Sowerby), and partly by the 
issuing forth of its spiral arms, which push asunder the 
valves like a wedge. 
Another genus was constituted and termed Orpicua, 
from the Patella anomala of Miiller, Zool. Dan. vol. i. p. 
14. t.5. The under valve is very thin, and fixed ; the up- 
per is orbicular, and depressed. It is a member of the same 
family as the preceding in the system of Lamark. 
32. Denrattum.—This very natural genus of Linnzeus 
has undergone no alterations, nor has our knowledge of the 
inhabitant been satisfactorily enlarged. The Dentalium im- 
perforatum, trachea, and glabrum of Montagu’s Testacea 
Britannica, do not accord with the essential character of 
the Linnzan genus in being “utraque extremitate pervia.” 
33. SerpuLA.—This genus has undergone several changes 
in the hands of modern conchologists. The |S. seminulum 
has been transferred to the genus miliola, and the S. filo- 
granum to the tubipora. — Besides these trivial alterations, 
the character has been greatly circumscribed, so as only to 
include shells which adhere to other bodies, and are tubu- 
lar, entire and flexuous, with a simple mouth, as represent- 
ed by the S. contortuplicata of Linnzus. The species 
which are regularly spiral, discoid, and fixed, as the S. spi- 
rorbis, now constitute the genus Sprrorgis. But as there 
are both dextral and sinistral shells with this character, 
the Herrroprsca may receive the reversed species. 
The genus VermicuLarra is formed from those species 
which, in appearance, resemble the spirorbis, but are not 
adherent, such as the S. lumbricalis. The shell at the 
mouth is, in general, somewhat produced. There are two 
or three minute British shells of this genus. 
The genus Srr1quarra, represented by the S. anguina, 
is distinguished from the serpula by a longitudinal, lateral, 
subarticulated fissure, which extends the whole length of 
the shell. 
The genus Penicrxu0s is formed from that curious shell 
the S. penis, and well known by the name of the watering- 
pot. The disk is perforated by a number of small holes. 
34. Trrepo.—From this genus, now considered as a bi- 
valve (the tube being regarded as an accessory covering), 
the Fisrutawa, of which 7. elava of Gmelin is the repre- 
sentative, has been separated. The external tube in this 
genus is closed at the posterior extremity, while in teredo 
it is open. The S. polythalamia forms, according to La- 
mark, the genus SerTaria, and the two genera XyLopHa- 
GA and CLAvAGELLA have more recently been instituted. 
35. SapetLa. This last genus of the Linnean vermes 
testacea has been degraded from its rank in conchology. 
The covering consists of agglutinated particles of sand and 
fragments of shells, and bears no resemblance to the testa- 
ceous coverings of the true mollusca. It is now placed in 
company with the terebella, and the three preceding Lin- 
nean genera among the ANNELIDEs. 
In the preceding brief review of the Linneean genera of 
shells, the reader will probably have been astonished at 
those changes which have taken place. » In this country we 
are so much accustomed to the artificial method both in 
zoology and botany, that we often reject, without sufficient 
consideration, the improvements which the study of the natu- 
ral method has suggested. In the time of Linnzus, per- 
haps, the genera of the shells, with a few exceptions, were 
sufficiently numerous and commodious to embrace all the 
known species ; but since the science has been cultivated 
with more zeal, in consequence, we must say, of the intro- 
duction of the natural method, the number of species has 
increased tenfold. New genera and orders, and other 
conventional divisions, have been formed, suited to the 
state of improvement of the science. The merit of all 
these improvements did not originate with Bruguiére or 
Lamark, whose names we have so often had occasion to 
mention. Many of the modern genera may be traced to the 
systems which prevailed before the days of Linnzeus; sys- 
tems which the Swedish naturalist, in his desire to simpli- 
fy, when simplicity was impracticable, too incautiously dis- 
regarded. 
CHAP. III.—-SYSTEMATICAL DISTRIBUTION OF THE 
MOLLUSCA. 
Tue successful methodical disposition of molluscous ani- 
mals, could not have been accomplished previous to labours 
similar-to those of Cuvier and Lamark; or until the shell 
and the contained animal were studied as connected objects. 
When thus contemplated, molluscous animals admit of ar- 
rangement into two great classes, or divisions, which may 
be distinguished from each other by well-defined characters. 
In the one, the presence of a head may be recognised, to- 
gether with eyes, and even ears in some of the groups. In 
the other, containing animals much less perfect in their or- 
ganization, there is no head, neither vestige of eyes or ears 
in any of the species. The former have been termed JMo/- 
lusca Cephala, the latter Mollusca Acephala. This ar- 
rangement was first employed by Baron Cuvier, and after- 
wards by Lamark and other modern systematical writers. 
349 
Sqm 
