TEREBRATULÆ (ANOMIÆ, Lin. see that article Encycl.) have been supposed not to exist now but as petrified shells. This, however, is a mistake. The anomia is an inhabitant of every region, and has existed in every age. As many terebratulae were caught by Perouse's people during his voyage of discovery, and as Lamanon the naturalist thought they should be considered as a genus by themselves, he has given us the following description of the anomia, or, as he calls it, terebratula, on the coast of Tartary:
The length of the shell varies from six to twenty lines, and its breadth from five to eighteen; there are, however, considerable varieties of proportion between different individuals, besides those arising from the different ages of the animal. It would be improper, therefore, to distinguish the various species of anomiae by the proportion of their shells. The wavy lines on the edges of the shell are equally defective, as di-
flinative characters; for our author observed in the same species the shell approaching or receding indifferently from the circular form, and, in some, the edges of the valves are on the same plane; whereas in others, one of the valves forms a salient angle in the middle of its curve, and the other a re-entering angle.
The shell is of a moderate thickness, about that of a common muscle; it is somewhat transparent, convex like the cockle: neither of the shells is more sensibly arched than the other; that, however, which has the spur, is rather the most so, especially in the superior part.
On the surface of the shell are seen a number of slight transverse depressions, of a femicircular wavy form, which reach the part where the shell ceases to be circular, in order to form the angle which supports the summit.
These frize are covered with a very thin and slightly-adhering periosteum; in some specimens there are from one to three shallow broad depressions, radiating insensibly from the centre of the shell, and becoming more marked as they approach the edges, where they form, with the corresponding parts of the other shell, those salient and re-entering angles which have been mentioned. The periosteum is rather more firmly fixed on the latter angles than on the former.
The shells are equal in the rounded part of their edge, and close very exactly; however, towards the summit, the spur of one of the shells reaches considerably beyond the other shell, consequently they are unequal, as in oysters.
The spur, or summit, is formed by the folding from within of the edge of the shell, and the elongation of its upper part. The folded edges form an oval aperture of a moderate size, through which the animal extends the muscle, by means of which it attaches itself to other substances. This shell is not, therefore, perforated, as its name of terebratula would seem to imply, the opening not being worked in one of the shells, but formed by the elongation of one shell, the folding in of its edges, and the approach of the other shell. The summit is not pointed, but round.
The ligament, as in the oyster, is placed between the summits, and does not appear on the outside; it adapts itself to the pedicle of the animal. As the summit takes up a considerable part of the shell, the valves are only capable of opening a very little without running the risk of being broken. It is very firm, though slender, and not easily to be discovered, being fixed in a small groove, which is filled up when the shell is shut by the corresponding part of the opposite shell. This ligament preserves its texture, even for a considerable time after the shell is emptied and become dry.
Oysters are without a hinge, the teeth which form it in many other shells not existing in them. The anomia has been considered as an oyster, because its hinge or teeth have not been examined: they are not visible indeed in the fossil specimens; but in opening them when alive, the teeth composing the hinge are sufficiently visible, being even much larger than in the greater part of bivalve shells. The fossil terebratulae are almost always found with their shells closed; whereas the other bivalves have usually theirs either open or separated: the reason of this seems to arise from the nature of the hinge, that of the anomia not allowing it
to separate, and the ligament, which is very tight, contributing to keep the two shells united. The teeth which form the hinge of the anomia approach very near to those of the spondyle, described by M. Adanson. In this last they are formed by two rounded projections, and in the anomia by the same a little elongated. It is above these teeth that the ligament is placed in the larger shell: there are between it and the teeth two cavities, one on each side, which serve to receive the teeth of the other valve. The teeth of the larger shell have, besides, a slight projection, which fits into a longitudinal furrow in the other shell in front of the teeth.
The substance which covers the inside of the shell holds, as in oysters, a middle place between naere and the interior substance of shells, which are destitute of it. The degree of its lustre, polish, and thickness, varies with the age and circumstances of individuals.
The colour of the teeth is always white; that of the outer surface of the shell verges more or less to the ochry red, especially on the border. The inside has also a very slight tint of this colour, on a varying greyish-white ground.
There is visible on each side of the shell the impression of two very distinct tendons; a circumstance which forms a very essential difference between this genus and that of the oyster: this latter having only one tendon arising from the middle of the body. The impressions of the tendon in the largest shell are oblong, situate near the summit, and hollowed; each of them has curved transverse ridges, divided into two parts by a longitudinal furrow, representing the wings of certain insects. In the other valve the insertions have a different form: their situation is the same, but they are very irregularly rounded and encompassed by two fulcations, which are separated from each other by an intervening ridge, and then are continued in a right line towards the opening of the shell as far as about two thirds of its length. That part of the summit of the shell along which the pedicle of the animal passes, is longitudinally striated in the larger shell, of which the middle stria is the deepest: the longitudinal striae are divided into equal parts by a transverse depression. There are no similar marks on the other shell.
Our author dissected the animal itself, and found what he calls the manteau of the anomia, formed of a very fine membrane, lining the inside of both shells, and containing the body of the animal. Its origin is of the same breadth as the hinge of the shell, whence it divides into two lobes, lining both the shells: it forms, therefore, only a single aperture, terminating at each end of the hinge, and of the same breadth with the interior surface of the shell: it appears to have only one trachea, which is formed by the two lobes of the manteau.
Our naturalist having opened the shell, divided the ligament as delicately as possible, unfixed the hinge, and detaching from the larger shell the lobe of the manteau, turned over the body of the animal. This operation exposed to view the large muscles which adhered to the shell; they are soft, membranous, and, as it were, fleshy on the inside, being covered with small sanguiferous glands. From the lower part of each muscle there proceeds a pretty strong tendon, which reaches to the extremity of the manteau; they run parallel to the edge of the shell, and at a considerable distance from each other; and are each enclosed in a sort of flattened sac, of
the shape of a ribbon, which is filled with a red viscous matter. It appears that the place of insertion of the muscles, as well as the muscles themselves, which extend along the lobe of the manteau, furnish real blood, which is contained in three small fleshy red glandular bodies of unequal size, which are visible after having taken off the muscles; perhaps these constitute the heart of the animal.
The muscles which are inserted into the other shell are also divided into several parts: some are seen extending along the corresponding lobe of the manteau; many others rise up in a kind of tuft, which is fixed into the shell above: some again subdivide into such minute ramifications as not to allow of tracing their course, even with the assistance of a microscope; but others, more apparent, contribute to the formation of the pedicle which passes through the opening left between the two shells, is connected to each of them by several fibres, and fixes itself to some external body, principally to other bivalves. The muscles of the anomia have therefore three attachments, namely, to the inner surface of each shell, and to some external body.
The form of the pedicle is cylindrical, being enclosed in a muscular substance, which contains several fibres; it is from a line to a line and a half long, and two thirds in diameter. It adheres so forcibly to different substances, as that the animal, and all the muscles which contribute to the formation of the pedicle, may more easily be torn through than the pedicle detached from the place of its adhesion. The glutinous substance which connects them to each other, resists even the heat of boiling water. It is by means of this pedicle that the animal raises its shell so as to be, while in the water, in a position inclined to the horizon. The smallest valve is always the lowest, being that upon which the animal rests; the superior one being the larger, and serving as a covering. Our author thinks the animal has the power of loco-motion.
After raising the lobe of the manteau he observed the ears. They are large, composed of two membranaceous laminae on each side, of which the superior is the narrower. These laminae are connected to each other by a thin membrane, so as to form only a single pouch. They have on their edges long fringes, which hang loose upon the manteau; but a very remarkable circumstance is, that their ears are supported by little bones like those of fish. The form of the ears is that of an arch; they are separated from each other on their lower part, where the fringes are the longest; so that the two ears on one side are perfectly distinct from those on the other side. The commencement of the ears is at the teeth of the hinge.
Between the ears are situate the stomach, œsophagus, and mouth; the whole forming a triangle, of which the mouth is the base. It is placed at the side of the hinge, and consists of a large transverse opening without lips or jaw-bone. The œsophagus is very short, but is capable of elongation when the animal opens its mouth. The stomach, which is of the shape of a pointed sac, is connected by a membrane to the bones of the ear. On opening the stomach, he found a small shrimp half digested.
At the bottom of the stomach is seen the intestine, of which it is, as it were, a continuation. It is extremely short, not exceeding half a line in a shell fifteen
Terebratula, Ternai.
lines across, and is composed of a very slender membrane. The excrements are discharged upon the lobes of the manteau, but they are easily thrown out by the motions of the two lobes.
The little bones of the ears, already mentioned, had not formerly been observed in any of the testaceous animals; whence the terebratulae approach nearer to fish than the inhabitants of any other shell. In the anomia which are preserved in cabinets, there is found only a very small portion of these bones, whence they have obtained the improper appellations of tongue or fork, which indicate only the form of the fragments, and not their use.
The small bones of the ears are composed of several pieces, the principal of which is of an oval form; it springs from the side of the hinge, of which it appears to be a continuation; thence it extends about two-thirds of the breadth of the shell, where it is reflected, and rests against the upper part of the fork, to the branches of which it is united by a simple superposition; a kind of articulation very common among the numerous small bones that compose the heads of fish. The fork extends from the summit a little more than one-third of the breadth of the shell: it is formed by a pivot which divides into two long and pointed branches; these are remarkably brittle, and support the extremities of the bones of the larger ears. The lamina, which composes a second set of ears, rests upon a curved bone, which on one side is attached to the inferior internal part of the bone of the larger ears, and on the other reaches to the side of the mouth of the animal, where it is united to another flat little bone, which is applied to a similar bone on the other side. These last little bones are exactly below the membrane which forms the mouth. All these bones are flat, very brittle, and surrounded with fibres and membranes. By their articulations the ears are enabled to move; they also support the body of the animal, which touches neither of the shells, but remains between them as upon trellises. The space between the branches of the bones of the ears is filled up with a transparent firm membrane; at the base of the fork is a similar one, and a perpendicular partition dividing the space occupied by the body of the animal from the rest of the shell. There are two orifices in this membrane communicating with the space between the two lobes of the manteau, and which serves as a trachea; for we have remarked, in the description of the manteau, that the two lobes are entirely separated from each other, and therefore do not form a real trachea.
From this description, it follows that the anomia ought to be separated from the genus oyster, since it has a toothed hinge, several ligaments, and an interior organization wholly different; neither ought it to be confounded with the cockle, the shells of which are both equal, and are destitute of any fenestole periosteum, without reckoning other differences. It has still less analogy with the other bivalves, and therefore ought to constitute a peculiar genus; the species of which, both fossil and living, are very numerous.
See Plate XLIII. where fig. 1. is a front view of a terebratula of middle size. Fig. 2. is a view of the internal structure.—A A, laminae of the superior ears.—B B, laminae of the inferior.—C, the stomach.—D, the anus.—E E, the manteau.—F, the cephalagus.