MOLLUSCA. 
Mollusca. rise of temperature, however, especially if accompanied by 
—-~>’ moisture, excites their revival and motion, and the lid be- 
comes detached. If we bring, for example, the Helix ne- 
meoralis, from its cold abode, and in an apparently torpid 
state, with the mouth of its shell closed by the lid and ad- 
hering to a stone, into.a warm apartment, it will speedily 
revive, especially if it be moistened with a little water, 
burst open the lid and begin to crawl. If the animal be 
again exposed to a low temperature, it again secretes ma- 
terials for a new lid, and speedily returns to its slumbers or 
inactivity. The first formed opercula, in these animals, al- 
ways contain a considerable proportion of carbonate of lime, 
a material which is found in smaller quantity in those of 
after formation. If the animal has revived frequently dur- 
ing the winter, the last formed opercula will be observed to 
consist chiefly of animal matter, and to be very thin. ‘The 
first formed winter lid of the Helix Pomatia is of the con- 
sistence of card-paper. 
All the land shelly mollusca appear to have the power of 
passing into a state of quiescence resembling torpidity, at 
pleasure, and independent of low temperature. Thus, even 
in midsummer, if we place in a box specimens of the Helix 
hortensis, nemoralis aut arbustorum, without food, in a day 
or two they form for themselves a thin operculum, become 
attached to the side of the box, and assume a dormant con- 
dition. In this state of apparent torpidity they may be kept 
for several years. No ordinary change of temperature pro- 
duces any effect upon them, but they speedily revive if 
moistened or plunged in water. Even in their natural 
haunts, they are often found in this dormant state, during 
the summer season, especially when there is a continued 
drought. Thus the Helix nemoralis may frequently be 
observed several feet from the ground, and attached to the 
stem or leaves of plants, with the mouth of the shell closed. 
But it is not drought which influences these terrestrial 
shelly mollusca to assume this quiescent state. The Suc- 
cinea putris, a species in ordinary cases delighting in mois- 
ture, may readily be observed during summer in a dormant 
state, high on the leaves of the Water Flag, having retired 
from the moisture below. In the same manner, and from 
causes equally obscure, some of the marine shelly mollusca, 
as the Limpet, Periwinkle, and Trochus, may be observed 
in a quiescent state on the rocks, above the reach of the 
tide. A shower, however, in general excites the Succinea, 
as well as the Limpets and Periwinkles, to motion. 
CHAP. II.—PROGRESS OF THE SCIENCE. 
Naturalists have pursued a variety of methods in their 
examination of this important branch of Zoology, and have 
proposed systems of arrangement founded on very different 
principles, and marking different epochs in the science. In 
the methodical distribution observed by some, the form of 
the shelly covering has been exclusively attended to, while 
the organization of the animal itself has been overlooked, or 
even disregarded. A few have made the habits of the ani- 
mal, the groundwork of their system. Others have passed 
over the characters exhibited by the forms and structure of 
the shell, and have confined their attention exclusively to 
the form and structure of the contained animal. Lastly, 
there have been a few, who, embracing all the circumstan- 
ces connected with the shell, the animal, and its habits, have 
constructed systems at once natural and convenient. In the 
following sections we propose to consider these four classes 
into which the cultivators of this department of science may 
be distributed. 
Secr. I—Systems constructed from circumstances connect- 
ed with the characters of the Shell. 
The arrangement of the testaceous mollusca, according 
VOL. XV. 
to the different forms of the shell, is unquestionably the most 
q y 
329 
Mollusca. 
obvious and the most ancient method. It was first employed <= 
by Aristotle, the father of natural history, and even in the 
present day its admirers are warm in its praise. It is with 
great propriety termed the artificial method, because the 
characters employed have but aremote relation to the more 
important functions of the animal. This eminent philoso- 
pher had the merit of forming the great divisions of wnival- 
ves and bivalves. He likewise separated the turbinated uni- 
valves from such as have but an imperfect spire, and formed 
many genera, or rather families, still retaining the names 
which he imposed. 
The progress of the study of the shelly mollusca (the 
naked kinds being in a great measure neglected,) made very 
little progress for many ages after Aristotle had published 
his method of arrangement. Indeed, the first work of this 
sort which claims attention, is the Dictionarium Ostracolo- 
gicum of Major, which was published in 1675. To him we 
are indebted for the threefold division of shells into unz- 
valves, bivalves, and multivalves, and for an explanation of 
the terms then employed by conchologists. 
In the same career, but with more brilliant success, Lan- 
gius followed, and, in 1722, published his Methodus Nova 
Testacea Marina in suas Classes, Genera et Species distri- 
buendi. The following character is given of this work by 
the intelligent and industrious authors of the Historical Ac- 
count of Testaceological Writers. (Linn. Trans. vol. vii. 
p- 156.) “ After having noticed a multitude of mere de- 
scribers, we now come to an author who is not undeserving 
of the title of a scientific one, and whose system, so far as 
marine ¢estacea are concerned, (and of thesealone he treats) 
certainly glances at the great clue to simplicity, which was 
afterwards so successfully and admirably seized by the great 
reformer of natural history in general.”. But Langius de- 
serves more praise than is here bestowed upon him. : Be- 
fore his system appeared, the characters of the genera de- 
pended principally on the oué/ine, and were of uncertain ap- 
plication. He remedied the defect, by directing the atten- 
tion of conchologists to the form of the mouth in univalves, 
and to the structure of the hinge in. bivalves. Among the 
former, he constituted subdivisions of those ore supertus 
aperto, ore superius in canaliculum abeunte, and ore su- 
perius clauso. Amongst the latter, the circumstance did not 
escape him, that some of these shells are equivalve, others 
inequivalve ; some equilateral, others inequilateral. Hence 
he may be considered as the founder of the inferior divi- 
sions of the artificial method, and as having furnished, to 
modern conchologists, many useful hints, of which they have 
availed themselves, without, however, acknowledging their 
origin. 
Another important improvement was effected by Brey- 
nius inhis Dissertatio Physica de Polythalamiis, 1732, in 4to. 
This consisted in separating from the ordinary univalves, 
such shells as possess a cavity divided by partitions into se- 
veral compartments, and in forming them into a division, 
which he termed Polythalamium. These shells are now 
called Multilocular. 
The system of Tournefort, which was published by Gu- 
altieri, in his Index Testarum Conchyliorum que adservan- 
tur in Museo Nicolai Gualtiert, Philosophi et Medici, Flo- 
rentini, 1742, well deserves an attentive perusal. In his 
observations on the bivalves, now denominated the acephal- 
ous mollusca, he drew the attention of conchologists to an 
important character, and one of easy application, having ob- 
served that, in some genera, the valves do not close or unite 
all round, but that, at certain places, the shell remains in 
part open. Such shells, in modern language, are said. to 
gape. hs 
The system of the celebrated Linnzus, which ought now 
to be mentioned, is too well known in this country to de- 
serve particular notice. In many of the other departments 
2 
Mat 
Langius. 
Breynius. 
Tournefort 
Linneus. 
