within a dorsal cavity which opens by a wide fissure above the head. The sexes are separate; and, with a very few exceptions, the species are covered with a turbinate shell, whose aperture the snail has the power of closing, when retracted, with a lid or operculum attached to the dorsal and posterior part of the foot. They constitute by far the most numerous division of Gasteropods, embracing probably not less than eight-tenths of all univalve shells, and you can at once form a general idea of the order by examining the periwinkle and whelk. In the former of these you will observe that the aperture is entire, that is, the lip is continuous all round the mouth without break or fissure, while the latter has a canal at its lower part prolonged into a sort of spout. Shells formed like the first constitute Cuvier's family Trochoides, including, among others, the genus Trochus, now partitioned into many subgenera characterised by having a conical spiral shell with a quadrangular mouth pearly in the throat; the Turbo, in which the shell is turbinate and the aperture round; the Paludina, which has a similar but unsilvered aperture, and is a native of fresh waters; the Littorinæ, like this in character, but an amphibious race, frequenting the sea-shores, and indifferent whether it is submerged or left dry by the recess of the tide; the Nerita, known by its semilunar mouth and its depressed spire; the Phasianella, one of the prettiest genera, speckled like the pheasant or guinea-hen; and the Ianthina, which is non-operculate, but provided instead with a cellular float to support it at the surface of the sea. The second family of the order is named Capuloides, in which the shell is oftenest simply conical like a limpet, but sometimes there is a small lateral spire, the interior exposed, however, by the disproportionably large aperture, which is entire and never protected with an operculum. The genera are few and sparing in species; nor is there any common example, excepting I can quote as such the shell called the Hungarian-bonnet by collectors,-the Capulus of science. The third family, or Buccinoides, may be represented by the whelk, but it includes, in its wide embrace, many shells which have no canal but where the lip of the mouth is merely interrupted by a sinus or emargination. Hence we find in it the beautiful Cones (Conus) and Cowries (Cypræa)-the glory of every rich collection, and whose deficiency declares the poverty of mine-the Ovulæ and the Volutes (Voluta), inferior to none in attraction and sportive variety. The whelk (Buccinum) gives its name to the family as being its typical genus; and it is followed by the turreted Cerithium, the spinous rock-shells (Murex),

and the wide-cheeked Strombus, all of them being possessed by snails essentially alike in their anatomical structure and their leading habits.

7. TUBULIBRANCHES.-Similar in their organs of respiration to the Pectinibranches, from which Cuvier detaches them because of their being asexual or hermaphroditical,-a structure necessitated by their fixed condition; for, unlike all other Gasteropods, the shell of the Tubulibranches is immovably fixed to foreign bodies. This fixidity is accompanied with a considerable modification in the foot of the animal, which, its occupation gone, is reduced to a rudimentary state; and in the shell, which is spiral only at its apex, and ends in a long flexuous or straight tube, bearing a strong resemblance to the serpulous shell of some worms, with which, indeed, the Vermetus and Siliquaria, the principal genera of this order, have been often classified.

8. SCUTIBRANCHES.-This comprises a small number of Gasteropods likewise very similar to the Pectinibranches in the form and position of the branchiæ as well as in the general form of the body; but, like the last, they are hermaphrodites, and, unlike them, have the power of walking to and fro. Their shells are widely open, ear-shaped or patelloid, nonoperculate, so that they cover the animal like a buckler or dish rather than contain it. In their interior anatomy they make an approximation to the bivalvular mollusca, and their shell has a certain resemblance to a single valve of that class. The Halyotis, or ear-shell, the Fissurella, and Emarginula, separated from the Patella of Linnæus, are the principal examples.

Fig. 16.

9. CYCLOBRANCHES. Another small hermaphroditical family distinguished by their branchiæ, which, in place of being situated within a peculiar cavity, form a filamentous ribbon between the margin of the cloak and foot, nearly encircling the body. The genera are Patella and Chiton (Fig. 16), exceedingly dissimilar in their external appearance, the former being covered with a simple conical shell, the latter with a series of testaceous plates arranged along and across the back.

The fourth class of mollusca-the ACEPHALES-are all

aquatic animals and very numerous. The first order in it, named A. TESTACES, have the respiratory organs in the form of four broad leaves, a pair on each side of the body, which again is always contained within a bivalvular shell, that, in a few instances, has some additional pieces affixed over the hinge. The cockle, the mussel, and the oyster, are members

of the order, and make its leading characters at once familiar to you. The structure of the shell and of its hinge, and the habits of the animals, will be afterwards detailed; all I have at present to do is to indicate the names of the great families among them. First on the roll we find the Ostraces or oysters, whose mantle is entirely open in all its circumference, and the valves of the shell are closed by a single central muscle. The edible oyster is the true representative of the family, but Cuvier includes in it also the Pectens, the Anomiæ, the Spondyli, the Malleus or hammer-shell, the Avicula or pearl-oyster, the silk-spinning Pinna, the Arca, and a considerable number of fossil genera allied to one or other of those just mentioned. The second family is the Mytilacés or mussel-tribe, in which the mantle is open in front, but with a distinct aperture for the excrementitial discharges, and there are two adductor muscles to close the shell; one of them, however, is very small in some of the genera, and placed near the hinge. The family embraces the sea-mussels as well as those of our rivers and ponds, and some (Lithodomus, Coralliophages,) which have the faculty of boring into solid substances. The Camacées have the mantle close, perforated with three apertures; through one the foot is protruded, the next serves for the ingress and egress of the water necessary to respiration, and the third for the discharge of the excrements. The gigantic Chama is the best known of the family, which is a small one. conducts us to the Cardiaces, so called from the resemblance of the shell to a heart; they have two adductor muscles, a mantle open in front, and furnished besides with two separate tubular apertures at one extremity. The cockle (Cardium) is of this family, which is widely spread and abundant in variety, for the genera Donax, Cyclas-inhabitants of fresh waters-Tellina, Venus, and Mactra, the most numerous in species of all bivalves, with all the kinds allied to these, are reputed to belong to it. The fifth family is named Enfermés, because the mantle is open only at the anterior end or near the middle for the passage of the foot; and at the other end there are two tubes, often bound in one common envelope, and capable of being extended to a considerable length. The shell gapes more or less; that is, the animal cannot by any effort so aptly close the valves, but that an open space, generally at each extremity, remains. The Enfermés burrow in the sand to considerable depths, where you may dig up at low tides the Myæ and the Solens, or razorshells, the best specimens of their family. Cuvier, however, places in it also the Pholas, and the "fell Teredo" of which

It

I have had occasion to write so much, and which bore some in wood and some in rock, as you are well aware. At the end of the family we find a doubtful station assigned to the Aspergillum (Fig. 17), or water-pot shell, of which a perfect specimen will be a very desirable addition to your collection.

Fig. 17.

The ACEPHALES SANS COQUILLES, the second order of the class, are so dissimilar to the first, that Cuvier says it might be raised to the rank of a distinct class, if such were thought convenient. Their branchiæ assume diverse forms, but are in no instance divided into four leaves or lamella; on the contrary, they usually form a regular net-work on the inner surface of the tunic. Shell they have none, and consequently the conchologist disallows their claims upon his attention, but there is no order more interesting to the physiologist. The covering which stands in lieu of a shell is a coriaceous, or cartilaginous, or, sometimes, a merely fleshy coat, with two circular apertures on some part of it; one for the admission of the water containing all the necessary food and air, the other for the discharge of their excretions. There are two families. The first comprises the genera, of which the individuals are isolated and complete in themselves, though sometimes occurring in clustered groups; the second, those which are compound beings, a certain number of individuals being organically associated to constitute one body. The Salpæ-singular gelatinous animals which float in the sea in long ribbon-like chains or with every link detached,- belong to the first, where likewise are located the Ascidies, very unlike the former in all things, in the form and structure of the cloak, in the disposition of the branchiæ, and in their permanent fixidity to rocks, shells, or other foreign bodies. The compound family (les Aggrégés) is a very remarkable one, connecting the molluscans with the zoophytes, among which they were arranged, until the laborious. researches of Savigny proved the fallacy of their position : it contains locomotive genera, such as Pyrosoma, one of the most wonderful of created beings and genera permanently fixed, such as Polyclinum, with its animalcules disposed in star-like figures shining through a pellucid jelly.

The BRACHIOPODS have considerable affinity to the bi

valve mollusca. Like them, they are enclosed in a bivalve shell; but this is always fixed or cemented for life to one spot. They were crowded beyond calculation in former seas before the present race of marine mollusca had existence, but few have survived the cataclysm, and, from their present rarity, they are highly prized by collectors. Cuvier reduces them all to three genera; the Lingula, a sort of small mussel hanging from rocks in deep water by a cylindrical fleshy pedicle; the Terebratula, something like a Pecten or Cockle, with the under valve perforated to give passage to the pedicle by which the shell is attached; and the Orbicula, known by its circular figure and the inequality of the valves, the upper being elevated into a shallow cone, while the lower is flat and cemented to the rocks.

This letter, however dry and tedious in its details, will require more than one perusal to fix the names of the principal tribes on your memory; for unless these are familiar as household words our progress must be halting:

names are good; for how, without their aid, Is knowledge gained by man, to man conveyed?"