former having been founded on specimens taken from cavities in the root, and the latter on individuals obtained from the flattened frond of the Fucus on which the species usually takes up its abode: it is, indeed, by no means rare to find specimens in which the animal has moved from one of these positions to the other, and in such cases the apex of the shell represents P. cærulea and the base P. pellucida, or vice versa. The same change takes place with regard to P. miniata and P. compressa. I have in my collection a specimen of this latter shell which is P. miniata at the top, it having in its youth lived on the frond of a large Cape Fucus; it afterwards removed to the stem, and became compressed, and consequently is in this part the P. compressa; but by some accident it was again induced to change its situation, and, removing to a flat surface, the edge of the mouth expanded, and it became a second time P. miniata, or perhaps what may be called by some authors P. saccharina, as this also appears to be a conical variety of the same species. Lamarck has described a similar specimen; and Mr. Sowerby, in his Genera of Shells, has figured an example of this species, showing the two states. In like manner the Crepidula porcellana, when applied to a flat surface, has an expanded base and a flattened inner lip; but when adherent to a convex body, such as the stem of the sea-weed, or (as frequently happens) to the back of another shell of the same species, the animal being pressed into the cavity, the inner lip becomes concave, and the sides of the aperture are contracted in this state the shell is called by most authors C. fornicata.

When the shells of this family are adherent to irregular surfaces, they adapt their margins to the inequalities with which they meet. I have several specimens of Patella from the coast of Devonshire, having one or more processes on their sides, which fitted into holes in the rock to which I found them attached; and such changes are the more remarkable, as some specimens are seen constantly moving from place to place, whilst others appear to remain for a long time fixed in one spot, and even those that are thus stationary in the young state constantly elevate the margins of their shells when the tide is low. I have also a specimen of Siphonaria gigas exhibiting in a great degree a similar adaptation of its edges to the form of the rock on which it grew.

The substances to which attached shells become adherent, besides altering their general form, often change the character of their surfaces; thus, when fixed to ribbed shells,

like the Pectines, Cardia, &c., they are frequently variously ribbed, a circumstance which often takes place in the Common Anomiæ; and if attached to a Dolium, as in a specimen in the collection belonging to Mrs. Mauger, they even exhibit on their own surface the alternate broad and narrow ribs of that shell. In specimens of Crepidula adunca attached to Trochus doliarius (and inhabiting the same locality, they are not unfrequently so attached), the convex part of the former is marked with the ribs of the latter. Shells which are ribbed from this cause are, however, easily distinguishable from those which are naturally ribbed, the ribs in the former generally extending across or along the shell, and not radiating from its apex or nucleus, as in all shells the natural character of which is to be ribbed. In those which adhere to ribbed shells by the foot of their animal (as in most of the univalves), and are therefore capable of being moved from place to place, the young animal may have lived on a smooth surface, and have had a smooth shell; and may have moved, during its growth, to a ribbed body, producing ribs on the latter formed part of its shell, or vice versâ. In a specimen of Crepidula adunca, for example, in the British Museum, the upper half of the shell is smooth, and the lower half ribbed; and I have seen specimens, on the contrary, in which the apex was ribbed and the base smooth. This change of form has, however, misled some conchologists, for Dr. Bronn, of Heidelberg, has founded a genus under the name of Brocchia, on a specimen of Capulus, which had acquired a ribbed surface in consequence of having been attached to a Pecten or to some other radiated shell.

These alterations of form and surface are always most distinct in univalves and in the upper valve of bivalves. In the latter case, the edges of the upper valve being produced beyond those of the under, they are immediately moulded on the surface of the substance to which the shell is attached, whilst the under valve simply covers it over. This is well illustrated in the unique specimen of Hinnites gigantea in the collection of the British Museum, which must have been attached to some marine body having a Serpula growing upon it. There is merely an irregular convexity in the inner part of the under valve, but on the outer surface of the free valve is to be observed a representation of the whole form, and of almost the entire surface of the Serpula, in consequence of the edge of that valve, during each deposition of shelly matter, having rested on the worm-shell. In the collection of Mr. Lincolne is a specimen of an oyster

which, having been attached to a plank covered by a number of Balani and Serpulæ, has the upper valve marked with prominences, exactly agreeing in shape with the substances concealed beneath the under one. The edges of the valves of Barnacles being very closely affixed to the surface of the substances to which they are attached, it appears that they not only assume the form of the larger prominences, such as the ribs and spines of a shell, but also the most minute differences of its surface. A Barnacle in my collection, which had been fixed to a Scallop (Pecten suborbicularis), has not only the ribs of the latter marked across its valves, but the whole surface of the prominent part of the valves is covered in addition with minute rugosities, produced by the small projecting scales which cover the surface of the ribs of the Pecten, whilst the articulating portion of the valves is smooth, as in the common state of the species. In another similarly ribbed specimen the articulating portions are also ribbed like the rest of the valves; and in a third, which was found on a piece of roughly planed and loosely textured wood, the surface of the valves bears an exact resemblance to the grain of the wood on which the specimen was attached.

The thickness, the roughness, and the smoothness of the surfaces of shells appear to depend, in a great measure, on the stillness or agitated state of the water which they inhabit. The species of our own coast afford abundant instances of this: the shells of Buccinum undatum and B. striatum of Pennant have no other difference than that the one has been formed in rough water, and is consequently thick, solid and heavy; and the other in the still water of harbours, where it becomes light, smooth and often coloured. In the same way the specimens of Purpura lapillus, which inhabit sheltered situations, are covered with small arched scales, whilst those found in exposed places are thick and rugose. Lamarck, not being aware of this circumstance, considered the specimens in the first state as a distinct species, which he named Purpura imbricata. The English

shells of the genus Pinna (and doubtless the foreign ones. also) offer the same variations, which have given rise to similar subdivisions of species. Shells which have branching or expanded varices, like the Murices, are also much influenced by these circumstances; and hence many mere varieties, arising from local causes, have been considered as distinct species. Thus Murex angulifer is merely a Murex ramosus with simple varices; and Murex erinaceus, M. torosus, M. subcarinatus, M. cinguliferus, M. tarentinus and M.

polygonus of Lamarck, are all varieties of one species.* Murex magellanicus, when found in smooth water, is covered with large acute foliaceous expansions; but the same shell living in rough seas is without any such expansions, and only cancellately ribbed. In such situations it seldom grows to a large size; but when it does so, it becomes very solid, and loses almost all appearance of cancellation. Triton maculosus is very widely spread over the ocean in different temperatures and different kinds of seas; it consequently offers a multitude of varieties both in size and surface, all gradually passing into each other, and most probably produced by the operation of the foregoing causes. Indeed, a vast number of merely nominal species have been formed from the habit, too prevalent among conchologists, of describing from single specimens, or even from several individuals brought from the same locality, which would never have been considered as distinct had collectors kept in their cabinets a series of specimens found under different circumstances, and studied, on the coasts where they are found, the variations which shells undergo.

Those shells which are attached to rocks, either immediately by their outer surface, or by the intervention of a beard, are most acted on by these causes: thus the Anomiæ found in protected places are thin and transparent, while those which inhabit exposed situations are thick and nearly as opaque as the shell of an oyster; and the under valves of the Crania which are affixed to the branches of coral are very thick and solid, while those that adhere to the Pinnæ and other flat shells are so thin as to have been overlooked by conchologists, who have repeatedly described their upper valve as a species of Patella.

Boring shells are greatly influenced in regard to their size, thickness, and form by the hardness or softness of the rock in which they are found: thus the specimens of Pholas dactylus found in the soft rock of Salcombe, are large and thin, and are covered with beautiful, regular, arched scales; while those found in the hard rock are small, irregular, thick, with a very wide anterior gape and large dorsal valves, and closely wrinkled externally, but almost or entirely destitute of scales: and the Saxicavæ, found in hard limestone, are often curved and otherwise distorted, in order to avoid the harder parts of the rock during the process of boring.

*

"Varietates conchyliorum exclusi numerosissimas, Murices tamen frondosos admisi, quamvis inter se nimis affines."-LINNEUS, Syst. Nat.

Land shells are much influenced, as regards their size, by the temperature, altitude, and abundance of food, of the country in which they are found. Specimens of Helix arbustorum from the Swiss Alps, are not one half the size of those of the neighbourhood of London; the shells of Helix nemoralis and H. hortensis, found in the last-named locality, are not above two-thirds the size of those which occur in Portugal and in the south of France; and there is so much difference in size between individuals of Bulimus rosaceus found on the coast and on the mountains of Chili, that the latter have been described as a distinct species under the name of Bulimus chilensis. There would be no difficulty in multiplying examples of the same kind.

It is not so easy to determine the influence of climate on marine shells, although there is little doubt, from the great differences of size observable between specimens of the same species, brought from different localities, that it actually exerts considerable power. Indeed, I have been enabled to mark this difference in some of the shells found on our own coast. The specimens of Littorina petræa found on rocks with a southern exposure near Torquay, are larger than almost any others which I have met with in England; but the largest of this species that I have seen, occur on the part of the Breakwater at Plymouth next the sea, where they are much exposed to the sun. The latter are twice the size of any that I found on the northern face of that magnificent structure.

The colouring of many shells evidently depends on the degree of exposure to light, air, heat, and the action of the waves to which they may have been subjected. Thus, among the Patellæ and-Crepidulæ, those which are attached to the stems of Fuci or other round bodies, and are thus exposed on all sides, are of a dull colour, or nearly colourless. This is well exhibited in the specimen of Patella miniata before referred to, which had changed its place of attachment twice during its growth; the two portions of the shell formed while the animal was affixed to a flat substance being white, beautifully varied with bright red (the general colour of P. miniata), whilst the central portion of the shell is of a dirty yellow, with a few indistinct, reddish dots, like the ordinary specimens of P. compressa.* In like manner P. pellucida when obtained from the stems of Fuci is of a pale horn colour, whilst the same shell, on the leaves, is of a beautiful purple with longitudinal pale blue lines.

* Such exposed shells are very rarely bright coloured; but a specimen of Patella compressa formerly in the collection of the late Earl of Tankerville, but now in that of Mr. Lincolne, is coloured nearly as brightly as P. miniata.