Lattice shell

“ The entire form of the lattice-mantle is always symmetrically polyhedral, since its dorsal and ventral halves are symmetrically developed on both sides of the equatorial plane, and therefore correspond perfectly to the enclosed smaller halves of the central bivalve shell. The two valves of the lattice-mantle (dorsal and ventral valves) are never really united and grown together, but are in loose contact in the equatorial plane ”

“ The surface of the shell is often covered with radial spines, which are sometimes arborescent.
The central capsule seems always to preserve the same lentelliptical form (or triaxial ellipsoid) as in all other Larcoidea. With the increase of growth it encloses successively a larger part of the spiral cortical shell, but on the outside is constantly protected by the last turnings of the spiral, or by the lattice-lamella of the surface.
Synopsis of the Genera of Lithelida. ”

“ Between the shells, the stiffening was obtained by circular lattice girders, which consisted of angle rings attached to both the inner and outer shell at every plate joint, and of angle bars set radially and attached to the angle rings by gusset plates, and also by inclined angle struts which passed from the angle ring attached to the inner shell to the next higher ring attached to the outer shell. ”

“ Frequently one and the same species occurs in the form of crusts or of erect lamellæ, the identity being recognised by the characters of the individual cells.
Often a large number of species may be found on one shell. Two good examples of this are exhibited in Case A 2.
Case A 2. Lepralia pallasiana (Fig. 13) forms sub-circular vitreous patches on stones and shells; the cells are rather large, broadly oval, and with the front wall punctured with pores; the aperture is squarish and with a slight indentation on each side. ”

“ Pteropoda, containing Atlanta, Spiratella and Argonauta.
NUCLEUS. (A kernel.) Anything forming a centre around which matter is gathered. The nucleus of shells is the first formed part; the first deposit of shelly matter to which the successive layers are added; the apex of the spiral cone, of which most shells are composed. (See Cone.) The nucleus is formed within the egg in oviparous, and within the old shell in viviparous mollusca. It is frequently more transparent and light than the remainder of the shell, and sometimes falls off; when this occurs the shell is said to be decollated. ”

“ With respect to the absence of separate compartments, it should be remembered, that in the same species of Tetraclita we have individuals with the four compartments distinct and furnished with radii, and other individuals without any trace of a suture externally,—the outer lamina of shell (though not the inner) having become completely confluent all round. ”

“ In some shells the operculum is small, in others large, and progressing step by step we find some, such as the scallop and oyster, [Pg 9] with one side round, and the other (really an operculum) flat and as large as the shell; until we come to the perfect type with each valve the same shape and size. Then the operculum disappears, as in the limpet, and the covering shell becomes smaller and smaller, till in the Scutum ambiguum (Plate IX., Fig. 23) the shell bears about the same proportion to the animal that the little bonnet, fashionable a few years ago, bore to the lady that wore it. ”

“ The size of these pores and the breadth of their bars gradually increase towards the smooth surface. All six shells are connected by six thin three-sided prismatic radial beams, which are prolonged outside in six stronger spines, angular pyramidal, with smooth edges, as long as the shell radius. ”

“ The sheath is but little hollow on its under side. The diametric growth of the shell seems to be quite capricious; in the same group, some individuals thus increasing, and others not at all. When the radii are developed they are narrow and white, with their upper margins nearly if not quite parallel to the basis: their sutural edges are formed by large, irregular, branching teeth or septa. The alæ, also, have their sutural edges coarsely crenated; and when the shell increases by diametric growth, they are added to above the opercular membrane, so as to be nearly square at top. ”

“ The transitions of these shells, it must be confessed are so various as to render it extremely difficult, if not unsafe, to determine where one species ends and another commences, the difference in the colours affords no sufficient data, neither is the form of the shell, nor the height of the spire so uniformly certain as to constitute a precise criterion. ”

“ We may always know the different species by the forms and patterns of the shells in which they live. Some have very regular shells and prettily marked. These are usually rounded or arched on one side and flat on the other.
When you are looking for various microscopic objects in pond water you will often see these tiny shells [90] among the sediment on your slides, and if you will patiently wait a few moments you will soon see delicate, transparent arms slowly pushing out on every side like cautious feelers. (Figure 2, Amœba in Shell.—Carpenter, p. 445.) ”

“ Cellular structure is comparatively rare; it occurs most notably in Pinna, where the shell is composed of a vast multitude of tolerably regular hexagonal prisms (Fig. 162 B) . Membranous structure comprises all forms of shell which do not present a cellular tissue. Carpenter held that the membrane itself was at one time a constituent part of the mantle of the mollusc, the carbonate of lime being secreted in minute ‘cells’ on its surface, and afterwards spreading over the subjacent membrane through the bursting of the cells. ”

“ In this case the ellipsoidal shell is composed of two or three (rarely more) concentric shells. Constantly one or two of these fenestrated shells are enclosed in the central capsule, and may therefore be called "medullary shells"; and one or two (rarely more) lie outside the central capsule, "cortical shells." The inner medullary shells (one or two) are either spherical or ellipsoidal; the outer cortical shells (one or two, rarely more) are always ellipsoidal. All concentric shells are connected by radial beams. ”

“ Afterwards I view’d it every way with a better Microscope and found it on both sides, and edge-ways, to resemble the Shell of a small Water-Snail with a flat spiral Shell: it had twelve wreathings, a, b, c, d, e, &c. all very proportionably growing one less than another toward the middle or center of the Shell, where there was a very small round white spot. I could not certainly discover whether the Shell were hollow or not, but it seem’d fill’d with somewhat, and ’tis probable that it might be petrify’d as other larger Shels often are, such as are mention’d in the seventeenth Observation. ”

“ The successive layers, each having its own proper wall, are often superposed one upon another without the intervention of any supplemental or intermediate skeleton such as presents itself in all the more massive forms of the Nummuline series; but a deposit of this form of shell-substance, readily distinguishable by its homogeneousness from the finely tubular shell immediately investing the segments of the sarcode-body, is the source of the great thickening which the calcareous zones often present in vertical sections of Eozoon. ”

“ It is in accordance with certain variations in the shape and disposition of this mantle and collar, that shells assume such very different shapes. Sometimes the whorls or spiral ridges, are projected or thrown far out, and this produces the turbinated shell. Sometimes they scarcely rise above each other, but rather spread towards the sides, and then we have the discoid shape. Generally speaking, the whorls of a shell take a direction from left to right, but occasionally an opposite one ”

“ If we imagine a delicate discoid spiral shell of elegant form, marked with curved and diverging grooves, and inhabited by a tiny piece of clear jelly without organs, but capable of projecting the substance of its body (called “sarcode”) in the finest possible filaments through perforations in the surface of its shell, this first principal representative of the deep sea soundings is before us. In our prepared and mounted specimen it is the shell alone which we see; the delicate tenant has of course long since perished, and its beautiful envelope alone is left ”

“ We have delineated the specimen with all faults for the sake of greater accuracy, and from a persuasion that the Naturalist would prefer a correct representation from an undoubted original, to any figure in which its actual defects might have been amended by the pencil of the artist. The shell is depicted in its natural size, and it will hence appear, is little inferior in point of magnitude to the generality of those shells of the same species which are not of the reversed kind. The species is sometimes known to grow to the length of seven or eight inches, but such examples are not common. ”

“ When the animal has attained its full growth and the lip is finished off, the white band and the periostracum cells completely disappear, and only such cells persist as contribute to the internal thickening of the shell. Shell growth, in this species, is very rapid. If a portion of the pulmonary sac is laid bare, by removing a fragment of shell, at the end of 11⁄2 or 2 hours there may be detected a delicate organic membrane covering the hole, and strewn with crystals of carbonate of lime. ”

“ The surface of any shell, whether discoid or turbinate, may be imagined to be generated by the revolution about a fixed axis of a closed curve, which, remaining always geometrically similar to itself, increases continually its dimensions: and, since the rate of growth of the generating curve and its velocity of rotation follow the same law, the curve traced in space by cor­re­spon­ding points {554} in the generating curve is, in all cases, a logarithmic spiral. In discoid shells, the generating figure revolves in a plane perpendicular to the axis, as in Nautilus, the Argonaut and the Ammonite. ”

“ The sea-urchin shell consists of a membrane, stiffened into rigidity by calcareous deposits, which constitute a beautiful skeleton of separate, neatly fitting “ossicles.” The rigidity of the shell is more apparent than real, for the entire structure is, in a sluggish way, plastic; inasmuch as each little ossicle is capable of growth, and the entire shell grows by increments to each and all of these multitudinous elements, whose individual growth involves a certain amount of freedom to move relatively to one another ”

“ They seem restricted chiefly to a single stratum, scarcely half an inch in thickness, and lie, not thinly scattered over the platform which they occupy, but impinging on each other, like all the gregarious shells, in thickly-set groups and clusters. There occur among them occasional scales of Dipteri; and on some of the fragments of rock long exposed around the quarry-mouth to the weather I found them assuming a [Pg 451] pale nacreous gloss,—an effect, it is not improbable, of their still retaining, attached to the epidermis, a thin film of the original shell. ”

“ The discoidal forms, as the Rotalia, Rosalina, Cristellaria, &c. are involute, like the Nautilus, and divided by septa or partitions, which, like the enveloping shell, are perforated. The lobes of the body occupy contemporaneously every chamber, and are connected by a tube that extends through the entire series. In « 341 » the spiral form, as the Textularia, &c., the same structure is apparent.
Whatever the form of the body, the filaments always consist of a colourless transparent matter; they are capable of being elongated to six times the diameter of the shell. ”

“ The outer shelly portion of this body consists of horny layers, alternating with calcified layers, in which last may be seen a hexagonal arrangement. The soft, friable substance, that occupies the hollow of this boat-shaped shell, is formed of a number of delicate plates, running across it from one side to the other in parallel directions, but separated by intervals several times wider than the thickness of the plates; and these intervals [Pg 238] are in great part filled up by what appear to be fibres, or slender pillars, passing from one plate or floor to another. ”

“ Try, too, to get young forms as well as adult, for the young are often very different in appearance from the full-grown shell. Mark on them, especially on such as you have found yourself, the locality they came from, as it is very important to the shell collector to know this, since specimens common enough in one district are often rare in another. Either write the name of the place in ink on a corner of the shell itself, or gum a small label just inside it, or simply number it, and write the name of the place with a corresponding number against it in a book kept for the purpose. ”

“ Respecting the construction of the shell it may be observed that it is produced in the thickness of the mantle, or cloak-like covering which envelopes the body of the animal; the formation commences at the small end or spire, and gradually goes on, whorl upon whorl, as the still widening circles which give the ridgy appearance to most univalves, are called. ”

“ It is plain that this undoubted and obvious fact need not affect the logarithmic law of the shell as a whole; we have only to add a small constant to our equation, which becomes r = m + aθ .
There would seem, by the way, to be considerable confusion in the books with regard to the so-called “protoconch.” In many cases it is a definite structure, of simple form, representing the more or less globular embryonic shell before it began to elongate into its conical or spiral form. But in many cases what is described as the “protoconch” is merely an empty space in the middle of
Fig. ”