it occupies; but when it is short, is sometimes only slightly recurved, as in the Ancylus and Velletia), in order to maintain a similarity of terms for the same thing in these two forms, all the lines or grooves which pass from the apex of the cone to the mouth, and which are caused by some permanent modifications of the edge of the mantle, are called longitudinal or spiral, and all the lines which are parallel to the edge of the mouth of the shell, and which, in fact, are generally marks of its growth, or are caused by some periodical development of the margin of the mantle, are designated as concentric or transverse. Thus the striæ on the Cyclostoma elegans and Planorbis albus are longitudinal or spiral, and the lamella on Helix lamellata and H. aculeata are concentric or transverse.

But when we speak of the spiral shell as a whole, it is usual to call it short or elongate, according to the length of the imaginary axis on which the whorles are rolled; and when we speak of the length of the mouth, it extends from the line which forms the front to the hinder edge of the mouth, which, in the Ancylus, occupies the whole length of the shell: the breadth is the line which crosses this at a right angle.

It is equally easy to determine the natural position of the bivalves without the presence of the animal; for the ligament is always placed on the dorsal surface of the animal, and the mouth is placed on that side of the apex of the valve, or umbo, which is before the ligament. Consequently, if a bivalve shell is placed on the table, with its hinge-side uppermost, and with the ligament towards the observer, the shell will be in its natural situation, and the sides of the shell will agree with the sides of the observer.

It is to be remarked that Linnæus, and the naturalists of his school, described what is here called the front of the shell as the back, the left valve as the right, and vice versâ; and Lamarck, in general (but not universally), followed the same rule. The method above described is, however, so obviously correct, and other SO liable to confusion from the want of a every sound foundation, that it cannot fail, sooner or later, to be universally adopted.

SYSTEMATIC DISTRIBUTION.

MOLLUSCA is the name given to that large division of the animal kingdom which is characterised by having a soft fleshy body, destitute both of a bony skeleton supporting jointed limbs, and of a hard ringed skin.

They are covered with a muscular coat, called a mantle, endued with a glairy humour, and are generally furnished with a calcareous envelope called a shell, which is secreted by this coat for the protection of the body or of the more vital organs of the animal.

They are generally elongate, walking on a single central foot or disk, and furnished with one or more pairs of organs on the head and sides. Their nervous system, which furnishes the most distinctive character of the larger groups of the animal kingdom, merely consists of a certain number of medullary masses distributed to different parts of the body; one of the masses being placed over the gullet, and enveloping it like a collar.

This division of the animal kingdom is subdivided into five classes in the following manner :

A. Crawling on a foot placed under the body.

I. Gasteropodes, which have a distinct head, furnished with eyes and tentacles, and are usually protected by a conical spiral shell.

II. Conchifers.

Having the mouth placed between

the gills, they and the body enclosed between the two leaves of the mantle, which are covered with two shelly valves united by a cartilage.

B. Destitute of any foot.

III. Brachiopodes. Having the mouth placed at the base of two spirally twisted ciliated arms, between the two leaves of the mantle, which are covered with two separate shelly valves: they live attached to other marine bodies.

IV. Pteropodes.

Having a prominent head, with one or two pairs of fins on the sides of the neck, by which they swim about in the ocean. The body is often covered with a thin glassy conoidal shell. V. Cephalopodes, which have a large distinct head, furnished with eight or ten arms, by means of which they walk head downwards.

Linnæus refers all the animals inhabiting shells to five different genera; viz., Limax, Ascidia, Anomia, Clio, and Sepia. These genera may be regarded as the types of the classes proposed by Cuvier. Poli had, before his time, considered three of them as orders, under the names of Mollusca Reptantia, Subsilientia, and Brachiata. (i. 27.)

The terrestrial or fluviatile Mollusca, of which alone we have to treat in this little work, are confined to the two first of these classes.

The shell, which is peculiar to this division of the animal kingdom, may be seen covering the young animal in the egg, before it has gained all its organs,

E

as was observed by Swammerdam, and verified by the more extended observations of Pfeiffer, Turpin, and others. They are easily seen in the egg of the Limnai, Physa, Ancyli, and Bithinia, which have a transparent coat. (See Phil. Trans., 1833.)

The shells of the newly-hatched animals have been frequently considered as distinct species, and some very thin shells of land Mollusca, such as Vitrinæ, have been taken for the young of other well-known species, as H. hortensis. These young shells are easily known by their always being of a pale horn colour; the whorles are generally rather irregular, and enlarge very rapidly; and the apex of the whorl which was first formed is generally large and blunt, compared with the size of the shell. They are always destitute of colour, for the animal does not deposit the colouring matter until after it has been hatched; and it is therefore generally easy to distinguish in the young shell (and sometimes also in the adult) that part of the top of the spire which formed the shell of the animal when in the egg.

The shell is formed by the hardening of the animal matter, which is secreted by certain glands on the surface of the body, by means of chalky matter, which is also secreted by similar glands. It has been stated that the unhatched animal, very shortly after it is formed, begins to make its shell; and when it is hatched, deposits on the edge of the mouth of the little shell which covered its body in the egg a small quantity of the mucous secretion. This dries, and is then lined with some mucous matter, intermixed with calcareous particles; and when this hardens, it again places on its edge another very thin layer of the mu