from each other. The latter are sometimes buried in the mantle and hidden.

The anterior (a) and posterior (b) valves are semicircular and nearly equal; the intervening are subequal, oblong, and transverse with sloping sides. "The posterior valve, which is placed over the more important organs, is generally the most fully developed, and is the homologue of the shell of the Patella; while the others, which are arranged in front of it, are more imperfect; and the front one is the most rudimentary of the series."-J. E. Gray.

Fig. 92.

S

a

The external surface of each valve is divided into a centre and a right and left side. The apex (Fig. 92 a), more or less marked, occupies the centre, and looks towards the posterior extremity. Each side is divided into two areas by a

line passing diagonally from the posterior aspect of the apex to the anterior and lateral margin. They are easily distinguishable by being striated or granulated in opposite directions, the anterior area transversely, and the posterior area longitudinally. (Fig. 92.)

The inferior margin of the valves, imbedded in the mantle, is marked with neat small notches, variable in numbers. Their number may be counted by the porous lines which are to be observed on the inner surface of the valves diverging from the apex to the margin.

The lateral or transverse margin is deeply sinuated in the middle, and furnished with a thin prominent rounded process on each side. The central sinus (Fig. 92, s) is minutely serrulated, and the lateral lobes or wings (Fig. 92, 1, 1) are also frequently so charactered. These lobes can generally be seen from the under side without separating the valves. There are none on the anterior valve.

In the living animal these lobes are inserted into the substance of the mantle, and hence the valves are subject to its contractions. By this means the snail can roll its shell into the form of a ball more or less perfectly, like the wood-lice.

The marginal band varies in breadth and sculpture exceedingly. It is smooth or furfuraceous, shagreened, granular or scaly, hirsute or spinous. The granules and scales are arranged usually in quincunx. The spines are in most collected into tufts.*

*Gray in Ann. and Mag. N. Hist. xx. 69; and in Phil. Trans. 1847, p. 141-5.

II.

SIMPLE UNIVALVES UNIVALVIA ABSQUE SPIRA

REGULARI, LIN.

Simple univalves are not twisted spirally round an axis; -they are unwreathed shells, which are tubular when they taper to the apex and open at both ends; and patelliform when they are conical and hollow like a cup.

The tubular shell (Fig. 93) belongs to a single genus (Dentalium), that of itself constitutes an order in the class.

Fig. 95.

The conical shell (Fig. 94), although also pertaining principally to a peculiar order of Gasteropods, is not restricted to it. The shell embraces a wide series of similar forms. The simplest is a low cone with the summit obtuse, and the margin entire or angular; a second has the summit perforated; a third has a fissure on the posterior margin; a fourth has the apex pointed and recurved; a fifth has within the hollow a transverse partition; and a sixth has a cup affixed under the dome (Fig. 95). This is a very remarkable structure. Professor Owen assigns a reason for its creation in the following passage. "The necessity for such a superaddition is probably to be sought for in the more active locomotive powers of Calyptræa as compared with Patella; the foot in the former being, from its organization, adapted to more extensive and frequent contractions, would be liable

to affect the superimposed viscera if they were in immediate contact with it. A calcareous plate, the first stage of a

columella, is, therefore, interposed, which supports the viscera, and separates them from the locomotive organ."

III. SPIRAL OR TURBINATE UNIVALVES COCHLEÆ, LIN.

The shells of the Snail and Whelk are examples of spiral univalves. When they cover the body no adjective term is needed to express their position, but when they are imbedded in the mantle, the shell is said to be internal, and the Mollusk is said to be naked. All internal shells are white or horny, and they are only obsoletely spiral.

Fig. 96.

S

a

The annexed figure (Fig. 96) is that of a spiral univalve, in which a is the apex or tummit, s the spire, o the aperture, and b the base.

The spire consists of one or more whorls, a whorl being a complete revolution of the shell round its axis or columella. The shell figured has seven whorls.

From modifications produced by the plane on which the whorls revolve, the following figures are derived,

Discoid. When the whorls revolve on a horizontal plane and are applied close to each other, a flat or disc-like shell is the necessary result. In its volutions the shell enlarges insensibly from the centre or point of departure, and hence it follows that every whorl is larger in all its dimensions than the preceding one, and the centre itself is sunken on one or both sides. The sunken or depressed side is said to be umbilicate, when the depression is considerable. Ex. Planorbis.

Cylindric. When the whorls are nearly equal in diameter and rise on each other without any marked tapering. Ex. Pupa.

Conical or pyramidal. When the base is broad and flat, while the whorls form a spire graduated to a point. Ex.

Trochus.

Turbinate. When the whorls rapidly decrease in size and diameter, and form a conical oblique spire longer than the diameter of the body whorl. Ex. Littorina. (Also Fig. 96.)

Globose. When the whorls are few and scarcely raised above the body, so that all the diameters of the shell are nearly equal. Ex. Dolium, Helix.

*Trans. Zool. Soc. Lond. i. 210.

Turreted. When the whorls are many, and form a spire longer than three diameters of the body-whorl. Ex. Turritella.

Fusiform. When the shell is thickest in the middle or body-whorl, and tapers towards both the apex and base. Ex. Fusus.

Earshaped. When the spire is minute and the bodywhorl very large proportionably, and widely open. Ex. Haliotis.

Involute. When the whorls form a retroverted spire, which is bent in upon the body. Ex. Argonauta. Convolute. When the whorls are wrapped round the axis so as to embrace each other. Ex. Conus, Bulla. The aperture of a convolute shell is always parallel to its length.

The Cypræa (Fig. 97) is a convolute shell in our definition, but Linnæus describes it as being involute, the margins of the aperture being rolled in when the shell is fully grown and perfected. But the figure of the young and mature Cypræa is very different. The perfect shell, by an addition of calcareous matter to the edges of its lips, as goes on in the formation of every shell, would soon have the aperture entirely closed, as you will perceive on examining any species of that genus. To get rid of this difficulty, Bruguière and others have imagined that the animal

Fig. 97.

threw off the shell when it had become too small for his necessities, and then formed another more capacious, and better fitted for his ease. This theory labours under insurmountable difficulty; nor does it seem required by the circumstances of the case. The Cyprææ, in their immature state, have a very different form from that they have when full grown. When young, they are very thin and brittle, with an evident spire, and a wide aperture, the margins of which are not toothed and inflected, but plain and effuse (Fig. 98). They are then, in fact, convolute shells of the ordinary character, and are obviously enlarged, like all others, by the addition of matter to the

Fig. 98.

outer lip alone. But maturity brings with it a change in the organs of the animal. The lobes of its cloak become more developed, and ultimately very large; so that, one issuing from each side of the aperture, they can cover the shell, and meet in the centre of the back. These lobes are secretory organs, and pour out an abundance of lime in a vitreous state; and, by their motions spreading it over the outer surface, the shell is thickened, assumes a form totally different from its primary one, and dependent on the new development of the soft parts.

To the above forms most univalves may be reduced. Intermediate shapes are expressed

by prefixing the diminutive sub to the adjective word, as subglobose; or by combining two adjectives together, as ovato-fusiform.

The whorl which contains the aperture is called the bodywhorl. It is the last formed, and is only finished with the full maturity of the Mollusk. The number of whorls varies of course with the age of the individual, but it seems to be very uniformly the same in the individuals of the same species. Adanson, however, asserts, that in Purpura, Buccinum, and some other genera, the shell of the male has usually more whorls than that of the female, and is at the same time more gracile and elongated. The latter observation you may verify by examining our common whelks, in which it is easy to distinguish the female by the bulged contour of the body-whorl.

On holding a shell with the aperture towards you and the apex aloft, you will perceive that the whorls revolve from right to left. These shells are said to be dextral (Fig. 96). When the revolution of the whorls is in the contrary direction, the shell is sinistral.* Some dextral shells are occasionally sinistral, and such a specimen is prized from its rarity; but I do not remember to have seen any sinistral species with a dextral individual.

"You tell me, that it is generally concluded by philosophers, that the reason of the usuall turn of snailes from the left to the right, is the like motion of the sun, and that especially more nord-ward, there having not been hitherto discovered any in our parts of the contrary turn to the sun's motion. But this is not the only case, where they are out, who consult not the stores of nature, but their own phancy."-LISTER to RAY. Phil. Trans. 1669, p. 1014. +"My well-informed friend, Mr. Pratt, who obligingly arranged the shells in the Ashmolean Museum, tells me, that he knew a French naturalist who had contrived to obtain a breed of reversed snails, which he sold