By the deep sea

The Mollusca that actually live between tide-marks, though numerous as individuals, do not represent many species; but those of which we may find the recently-vacated shells, thrown up by the tide from greater depths, will total up to a considerable number. The bivalves must be sought for on sandy beaches and mud flats, especially at the mouths of rivers. Most of them are burrowers, excavating a way by means of the powerful foot with which they are provided. This instrument is well seen in the Razor-shells (Solen), or the Cockles (Cardium), where it reaches extraordinary development. Even where the animal lives far beyond our limits in deep water we can, by a little thought, get some notion of their habits by examining the empty shells that are cast up within the littoral zone by heavy seas. Those that are fresh and clean externally, though without any signs of wear from long washing among the shingle, may be safely regarded as burrowers that habitually lie beneath the sand or mud. These, too, will be found to have both valves of the shell almost, if not quite, equal in size and shape; whilst those which, like the Oyster and the Scallop, lie upon the sea-bottom, have very unequal valves, the under one being deeper and concave, whilst the upper valve is flat and more brightly coloured, to harmonise with its surroundings. Often, too, this exposed upper valve will be crusted with acorn-shells, Serpulæ, Sertulariæ, or seaweeds.

It may prevent confusion further on if we now say a few words by way of defining the parts of a bivalve shell, its latitude and longitude, and its relation to the animal whose vital activities produced the valves. The Spiny Cockle, or Red Nose (Cardium aculeatum) of our illustration, is on its back. If we were to take it, or any other bivalve shell, and turn it the other way, so that the hinge connecting the two valves was uppermost, we should have it in the natural position.

A bivalve mollusk is an inferior creature to a limpet or a winkle, because these have heads with eyes, but the bivalve has not. In the larval condition it has eyes, but by a retrograde movement like that of the cirripedes, it gets rid of these as useless in the life it is to live henceforth. But in spite of its want of a head, we know which is its anterior and its posterior end, its dorsal and its ventral surface; and with our knowledge of the relation of animal and shell, we are not troubled to open the valve to look at the creature, when we wish to describe the parts of a shell. It will be noticed that each valve curls over near the hinge and takes a form not greatly unlike a beak. This is more strongly marked in some species than in others; anyhow, it is popularly known as the beak, though it is technically distinguished as the umbone, or boss.

If these beaks have the slightest tendency to either end of the shell, it will be to the front, where we should expect the creature’s head to be, if it had one. This point made clear, by reference to the shell we have just picked up, we can say which is the right and which the left valve. The valves are hinged by a band of a substance that looks much like catgut. It is elastic in character, and is always pulling at both valves, so that the natural tendency of the shell is to gape open. But inside the shell there are, in most bivalves, two much more powerful bands of muscular fibres (the oyster has but one), which, by their tension, can slowly or suddenly bring the edges of both valves closely and tightly together, and hold them so for an indefinite period. You can see the marks where these muscles were attached, one at each end of the valve. Between these two marks (“muscular impressions”) there runs a colourless line marking the area to which the mantle was attached (“pallial impression”), but this line is often interrupted, towards the hinder end of the shell, by a bay or sinus (the “pallial sinus”).

The mantle is a delicate membrane on each side of the mollusk’s body, which has the power of forming the shell, to which it is attached save at the margins. The “pallial sinus” is caused by the syphons which are protruded at that end of the shell. At the other end, as shown in the figure of the Banded Venus, is the “foot.” The “syphons” are two delicate tubes, and if you were to put a living Venus, or other syphon-bearing mollusk into a glass of clear sea-water, and drop a little finely-divided indigo, or other colouring matter, in the immediate neighbourhood of these syphons, you would observe a stream of the minute colour-particles rushing into one of these tubes, and a stream of clear water issuing from the other. The inflowing stream passes between the leaf-like gills, or respiratory organs (“branchiæ”), where it is effectually strained, all solid matter being retained and passed on to the stomach, whilst the filtered water passes out through the second syphon. The length and form of these syphons differ in distinct species, but each kind is pretty true to its own type, and, consequently, the impression that it makes on the interior of the shell, taken in conjunction with the muscular and pallial impressions and the hinge-teeth, are a certain guide to the discrimination of species.

These are matters that are essential to one’s knowledge of the mollusca, and they must be learnt; but the few species we shall be able to mention in this chapter will be indicated more by their external shape, marks, and colouring. When so identified, the reader should strengthen his knowledge by a practical study of these internal impressions, and the characters of hinge and teeth.

This Spiny Cockle, or Red Nose (Cardium aculeatum), is not the Vulgar Cockle (C. edule), although it is much sought for food on its native Devonshire coasts. It is a very much larger species than the last-named, and gets its name of Red Nose from the brilliant hue of its long strong foot, which is at once a burrowing instrument and a leaping pole. By pushing its pointed end down into the sand, and then bending it into a hook, it can, by contracting the foot, pull the thick prickly shell down after it. On the other hand, by pressing its bent tip against some unyielding substance, it can use it as a spring, which shall suddenly send the shell flying through the water to some considerable distance. The Spiny Cockle is a creature of clean, sandy beaches, where it may be found at low-water, but only on the Devonshire coast.

The Common Cockle (C. edule) is very much smaller, its shell free from prickles, and marked merely with bold rounded ridges. It is more likely to be found where the sands are not wholly of sand, but contain a liberal admixture of mud. On some of our coasts it is exceedingly abundant, and in times of famine has saved populations from starvation. It is certainly on record that the people of the Isle of Barra, in the Hebrides, have been thus preserved many years ago, when all the people sought the Cockle on the great expanse of sands at the northern end of the island. “It was computed that for a couple of summers, at the time alluded to, no less than from one to two hundred horse-loads were taken at low-water, every day of the spring-tides, during the months of May, June, July, and August.”

The Cockles have gained their name of Cardium and Cardiaceæ from the fact that if the shell is viewed “end on”—the curving beaks, of course, uppermost—it will present the conventional heart-shape (Kardia, Greek—heart). Some nearly allied genera, exhibit a similar form, but narrower, and therefore not so suggestive of hearts; but the Heart Cockle (Isocardia cor) is more truly heart-shaped than the Cockles of the genus Cardium. It is about three inches across its longest diameter, very thick and heavy, and the beaks are so greatly curled that no one will be disposed to quarrel with the name, either of the genus or the species. It is a deep-water species, but in suitable localities the empty shell may be found washed in by gales. It is chiefly found on the west coast, and it is probable that its headquarters, in British waters, is around the Isle of Man.

Several of our most familiar bivalves are not very distantly related to the heart shells. There are, for instance, the Venus shells of which we have already given a figure of one species, the Banded Venus (Venus fasciata). It is a solid, heavy little shell, of some shade of brown, with broad bands of a lighter hue radiating from the beak. A series of strong ridges run parallel with the margins, or, as usually expressed, the ridges are concentric. The various species of the genus inhabit sand and gravel from low-water mark to a hundred and forty fathoms. The animal must be obtained by the dredge, but the empty shells are thrown up freely after storms. A much larger species is:—

The Warted Venus (V. verrucosa), a drab-coloured shell, with very rough and unequal ridges. In some specimens these ridges are so broken by radiating lines, that the ornamentation has the appearance of being warty. The various species of Venus have three strong hinge-teeth on each valve, and the inner edge, though at first sight smooth, is very finely “milled.”

The finest of these shells is the large, heavy Smooth Venus (Cytherea chione). It is a deep-water species, found chiefly on the southern and western coasts, where, in spite of its great weight, it is frequently washed up after storms. It is wonderfully smooth, inside and out; even the lines of growth are not high enough or sharp enough to take off this smoothness of the outside, which is coloured of a pale pinkish-brown tint marked by concentric lines of a lighter hue, and by much darker radiating bands. It is all very simple, but very effective. The inside is coated with white, and the muscular and pallial impressions are very strongly marked, though in no way interfering with the general plan of entire smoothness. The edges, too, are rounded and as smooth as the edge of a tea-cup. It is three and a half inches across the longest diameter of the shell, and its circumference, at right angles to the last measurement, is eight inches. The hinge-ligament is an inch long, and the teeth are very strong and prominent.

It is by no means a common shell outside the districts mentioned, but I have frequently found it on sandy shores in Cornwall, thrown up by storms, with the living animal still within. I believe most of the entire shells found on beaches have been thrown up whilst the animal is still in possession, and evidence upon this point may be obtained by examining the ends of the shells. It will be found that those which came to the surface with the animal are more or less chipped at the ends, where a Puffin, or other sea-bird, has cracked off a portion to enable it to prize the valves open; additional evidence will be found in portions of the muscular bands still adhering to the valves.

On the same sand and pebble beaches we shall find in greater plenty another of the Venus shells, the Rayed Artemis (Artemis exoleta). We presume that Linnæus, in giving this species its name of exoleta (Latin, worn-out), was struck by the fact that however fresh a specimen may be, it has the appearance of having been knocking about with sand and shingle for some time. The shells are white, with variable rays of pinky-brown (sometimes entirely absent), and finely and evenly marked with concentric grooves. In proportion to its size, it is a very thick shell; very round in outline, except that a piece appears to have been nicked out of the edge in front of the beak. When the shell is closed, these marks on the two valves, coming together, form a heart-shaped depression of a brown tint, and called the lunule.

The lunule is not peculiar to this species, but is shared by a large number of bivalves. It is well-marked in the Smooth Venus, but not so completely heart-shaped as in the Rayed Artemis. There is a finely-developed set of hinge-teeth, and the pallial impression is deeply sunk. A closely allied species, the Smooth Artemis (A. lincta), is smaller, not banded, and the concentric ridges are finer and less perceptible. It is this peculiar type of smoothness that suggested the specific name of lincta (Latin, sucked), its appearance being as though a specimen of exoleta had been sucked until smooth. Both these have a hatchet-shaped foot for digging into the sand. Great quantities of this bivalve are washed up in winter, and I have frequently come across a piece of rock protruding through the sand, around which there were dozens of these shells, broken or chipped, giving evidence, from their fresh muscles, that they had but recently been destroyed. It has reminded me of the favourite stone under the hedge, whereto the Thrush brings her snails to be hammered until the shell yields up its luscious contents. Artemis has met with a fate similar to that of the hedgerow snails, for her fortress has been broken by gulls, puffins, or even by ravens when winter has taught them not to be too particular about their food.

There is a group of Venus shells whose exterior is ornamented with concentric and rayed rounded ridges, in some cases strong, though regular and even, whilst in others they are but slightly perceptible; but their place is, to some extent, taken by colour. They bear the generic name of Tapes (Latin, tapestry) which is exceedingly appropriate, for the patterns of some species is very suggestive of tapestry and carpet. Especially is this so with the Cross-cut Carpet-shell (Tapes decussata), whose exterior looks like the back of a piece of tapestry carpet, both in texture and colour. The latter is of a nondescript drab, with occasional tinges of red and stain-like smears of bluish-grey. The ridges radiate from the beak, and they are nicely rounded, but their lines are by no means straight. They are cut across by fine concentric incised lines, which, with the grooves beside each rayed ridge, produce the cross-cut, or decussate appearance which suggested the name. The interior is dull white, like the surface of an enamelled card, the muscular and pallial impressions polished, and consequently very obvious.

The Virgin’s Carpet-shell (T. virginea) is smaller, the exterior very smooth, the ornamentation taking the form of shallow concentric lines. The colouration is a mottling of salmon-pink, with little specks of white showing through, and irregularly shaped spots of dark brown sprinkled sparingly over all. Viewed not too closely, it will be seen that the whole surface is divided between about six broad rays of lighter and darker tints. The interior is white and glossy, deepening to pink or yellow towards the beak and hinge.

The Golden Carpet-shell (Tapes aurea) is similar, but some shade of yellow takes the place of the pink in the last-mentioned species.

The Pullet Carpet-shell (T. pullastra) is broader from the hinge to the edge of the shell, in proportion to its length at right angles with that measurement. Its name, pullastra (Latin, a pullet), has evidently a relation to its colouring, which is similar to that of virginea, though darker. If the two are compared it will be found there is a further difference in the fact that whilst virginea can scarcely be said to have any radiate-grooves, pullastra is covered with them; but they are not appreciable to the sense of touch, and scarcely so to ordinary eyesight, unless special attention is drawn to them—they are so exceedingly finely cut. Inside, the shell is white, that part lying between the impressions and the hinge being dull like the whole interior of decussata; but the impressions and the outer margin are polished.

The Tapes animals spin a byssus like the Mussel; they burrow in the sand at low-water with their thick fleshy foot, or spin up to the roots of Laminaria and other seaweeds among the low-water rocks. Around the shores of the European continent they are used as food, but do not appear to be so utilised in Britain.

The Scallops are familiar to all in a general way, from the frequency with which one species occurs on the fishmongers’ slabs. This is the largest British species, and is generally distinguished as the Common Scallop, Quin or Queen (Pecten opercularis), a deep-water species, whose valves are frequently washed up on the beach. They occur in beds, but are not fixed like the Oyster; on the contrary, by the sudden closing of their valves and the consequent rapid expulsion of water, the shell shoots hinge foremost through the water to some considerable distance. The young ones can attach themselves by a byssus to the rocks, as is done by the Mussels, Carpet Shells, and others. It is a peculiarity of the Pectens that they have a pair of “ears” to the shell, the edges of which afford a good foundation for the hinge ligament, whilst in lieu of hinge-teeth to keep the valves firmly together when closed by the muscles, the corrugations of the valves extend right to the margins, and the ridges of the right valve fit into the furrows of the left valve and vice versâ. It will be noticed that these ears are not a good pair—one is always larger than the other, and the smaller one is popularly supposed to have been broken; that, however, is a mistake, the Pectens are built that way. The most prominent ear is always the front one, and below that of the right valve there is a notch where the byssal threads issue. In the Common Pecten these ears are much more nearly equal than in the others, whilst in P. varius there is a great contrast in the size and shape of the two, and in P. tigrinus one is almost absent altogether.

The Common Pecten is sometimes dredged for, but as a rule it is avoided by the fishermen, on account of the risk to their nets and the small price realised for the mollusks after they have caught them. It will be noted more conspicuously in this species, on account of its size, that the right valve, which is the lower one when the creature is lying on its bed, is far more convex than the left or upper one. It is exceedingly variable in colour.

The Variable Pecten (P. varius) carries out its name to the letter, for out of a score one could scarcely find two that agreed in colour and the disposition of the markings. Their usual tint will be found among the red series of the chromatic scale; sometimes almost white with dark red blotches, at other times dark red-brown with faintly perceived cloudings of a still darker hue. The exterior is ornamented by about twenty-eight bold ridges radiating from the sharp beak, and each of these, as it approaches towards the other edge of the shell, gives off irregular spiny processes. There is a rare variety of this almost entirely white. P. varius is not content with using its byssus only in the days of its youth, but continues to do so, even when at full age; it may sometimes be found thus hung up to a rock, as shown in our illustration, or attached to the roots of Laminaria.

A live Scallop of any species, in a glass vessel of sea-water, is a beautiful object. It will soon open its valves and exhibit the richly-frilled edges of its brightly-coloured mantle; this organ, in fact, has a double margin, the inner of the two finely fringed, and at its base a row of eye-like beads.

When prying curiously about the rocks at low-water, under the scrub of weeds and corallines, we are sure to discover little flat pearly shells which we shall almost as surely decide to be young oysters. They are a kind of oyster, though not edible oysters of the genus Ostrea, but Saddle Oysters, of the genus Anomia (A. ephippium). Although they appear to be firmly cemented to the rock by the lower (right) valve, this is not really so. The thin blade of a penknife gently pushed beneath will move it off with the merest touch, for instead of being fixed by its whole under surface, it is merely attached by a muscular plug that passes through a comparatively large oval hole in the under shell, near the hinge, and sticks like a sucker to the rock. As it grows older it will probably alter its form, to adapt itself to things it comes in contact with, as its diameter increases. Small specimens, not so large as a threepenny-bit, usually have a colony of much younger individuals located on their upper shell.

Odd specimens of the Common Mussel (Mytilus edulis) will be found among the roots of weeds on the low-water rocks, but to obtain them in quantity one must go to a mud-bank, as at the mouth of a river; or they may be found clinging in masses to the wooden piles of piers and breakwaters by means of their byssus-threads. The first thing a mussel does on being placed in an aquarium is to attach itself to the side by this means. Possibly he will wander a little, by means of his foot, to make sure of the right spot upon which to cast anchor, but having settled that point and found that he has made the right choice, there he will remain.

A mussel is the best of all bivalves for aquarium life. It is true he is not very lively, and does not flit through the water like the young Scallops. He is anchored, and there he stays, simply opening his shell a little way and putting out the frilled edges of his mantle, with their openings for the inward and outward currents—the inward bringing both oxygen and food, the outward carrying off carbonic acid gas and other waste.

The large, thick, coarse-looking mussel shells we occasionally find on the sands, measuring five or six inches in length, belong to a different genus, and are called Horse Mussels (Modiola modiolus). The valves in question may not have come far, for the species occurs in sand and mud as near as low-water. But it will not be found moored to rocks and weeds by its byssus; instead, it burrows and weaves its enormous byssus into a nest with sand and gravel mixed up with the threads. They are said to be coarse and unpleasant tasting, so that they are not used as food, and hence the name, Horse-mussel; the prefix, horse or dog, before a popular name for animals or plants, denoting its worthlessness as food, the sole criterion of worth to the popular imagination being found in the answer to the query, “Is it good to eat?”

A very handsome shell, as well as a common one, is the Comb-shell (Pectunculus glycimeris), whose thick round valves may be found rolling on the beach, where they have been washed up from the zoophyte ground in deep water. It is very variable in its markings, and yet there is so strong a family likeness running through all its variations, that there is not the slightest difficulty in its identification. To the touch the exterior is quite smooth, though not glossy, but examined with a lens it will be found to be covered with very fine and regular lines running from the beak to the opposite edges. So fine and close-set are these, that a line an inch long, drawn across them at right angles, will cross about ninety of them. The valves are more or less covered with a colouring of rusty red, relieved by numerous long sharp wedges of white. These are on parts or the whole of the shell, sometimes so plentiful that there is no room for solid masses of the red colour, and it only shows in zigzag lines. It is difficult to get two shells that at all agree in the distribution of white and red, and even the two valves of the same shell will differ widely in this respect. The interior, also, exhibits characters sufficiently striking to prevent its mis-identification. There is a broad flange below the hinge, whereon are cut about twenty teeth, in two series. Immediately below the ligament is a smooth space, clear of teeth, but these are arranged in a row of about ten on each side of this space. As the shell grows the flange lengthens, and more teeth are added to the ends of the rows farthest from the beak; but those nearest the smooth central space are being rubbed down or absorbed at the same rate, so that the net increase is about nil. The free edge of the valves, internally, has a series of raised marks, like the tips of the teeth of a comb, and it is from these the creature gets its name. The pallial impression is much deeper than those of the muscles at each end of it, and it is uninterrupted by any sinus. This shell is about two inches in length.

We must not omit to mention a group of shells that are fairly common upon many shores, and are usually found among the bucketful the children have collected.

First of these is the bold Rayed Trough-shell (Mactra stultorum), more plentiful in the north than the south of Britain. The various species of Mactra are inhabitants of sand in deep water, but their shells are freely cast up on the shore. These are smooth, except that the annual periods of rest from shell-making is plainly marked in deep concentric grooves. Like that of the Spiny Cockle, the foot of Mactra can be extended and used like a finger, and also as a leaping pole. They are destroyed in great numbers by star-fish, and many empty valves may be found with the clean round boring that shows the animal fell a victim to the whelk. M. stultorum is usually coloured some shade of brown, with a number of white bands radiating from the beak. The hinge arrangements in this genus are worthy of note, as indeed they are in all the genera, and must be carefully studied by anybody who wishes to have anything more than the merest superficial knowledge of conchology.

In the Trough-shells the ligament of the hinge is short and thick, and contained in a spoon-like cavity in each valve. Immediately in front of it there are two shelly teeth, joined above in the form of a Ʌ, and from each side of the beak there runs off a strong ridge-like tooth, the surface of which is “milled” like the edge of a shilling or a sovereign. The Elliptic Trough-shell (M. elliptica) is not so triangular as M. stultorum, and is without the white rays. The Cut Trough-shell (M. truncata) might be appropriately styled the hatchet shell, for its truncated ends give it a very close likeness to the head of a hatchet.

Related to the Trough-shells are the Otter-shells (Lutraria), of which we have two species. They burrow in the mud and sands, of estuaries especially, and are found from low-water to about twelve fathoms. Having found a complete, though empty, shell, you will be surprised to discover that it will not close properly, and you not unnaturally suppose that you have got hold of a malformed specimen, whose shell has got a twist somehow. That, however, would be a mistake, as you would find when other specimens came in your way, and you found they all had the same objection to closing at the ends. From one end, when the creature is alive, protrude its united syphons, large and thick; and from the other end is thrust out the useful “foot,” with which its burrowing is effected. Where you happen to find the usually broken valves of the Otter-shells, it is worth while to dig in the muddiest spots thereabout at extreme low-water, and you will probably be rewarded with perfect specimens, and have the greater satisfaction of seeing the living creature within.

Then there are the Tellen-shells (Tellina), a bright and delicate-looking group, with shells that appear as though they had been subjected to considerable pressure. They are finely grooved with concentric lines, and decorated with broad bands of pink. One of the most plentiful of these is the Thick Tellen (Tellina crassa), in which the pink bands radiate from the beak across the shell. Thick is a comparative term, and is so used here, for the shell, as compared with a Mactra, for instance, would be considered rather thin; but in contrast with other Tellens, it is solid and substantial. The interior is delicately tinted with pink or orange. The pallial sinus is large and rounded. The Fragile Tellen (T. tenuis) has thin shells that are very easily broken. Its surface is very smooth, of an orange tint marked with bands of pink and white. There are half-a-dozen other British species. The Tellens burrow slightly in sandy mud, frequently in shallow water. They may be dug for on a suitable beach between tide-marks, though their range extends to about fifty fathoms.

Somewhat similar to the Tellens in their delicacy and style of ornament are the Sunset-shells (Psammobia), so called on account of the crimson patch around the beak, from which rays of a similar hue run off to the margin. If the shell is so placed before you that the beak is downwards, these rays suggest the far-reaching rays from the sun that streak all the western sky, when Sol dips below the horizon for the night. There are four British species. The two ends of the shell are nearly equally rounded, but in an allied genus—

The Wedge-shells (Donax), the hinder end is much more acute than the front, so that their popular name is very suitable. They have a suggestion of sunset rays, too, but not so strong or so symmetrical as in Psammobia. The most familiar species is the Common Wedge-shell (Donax anatinus), which may easily be distinguished from the others by the milling of the interior edge of the valves. The Polished Wedge (D. politus) may be equally well separated by its superior gloss, and by a white band which runs backward from the beak.

Then there are the familiar Razor-shells (Solen) that must be dug out of the sand at low-water; and quick work you will find it, if you succeed in catching any specimens. Very good examples may often be picked up on a wide sandy beach, but minus the animal. They are sought for food, and the professional catchers are very expert in their movements—they need to be, or the business would not pay a dividend. Everybody knows the razor-handle-like shells of Solen siliqua, if they have no acquaintance with the animal. They are flattened cylinders, widely open at each end for the extrusion of the foot and the syphons. The hinge is near the front extremity of the shell, the ligament in a full-grown specimen measuring an inch and a half. There are two teeth in each valve, though some have three in the left; but it is difficult to pick up empty shells in which the teeth are intact. The Razors spend all their lives buried vertically in the sand. When the sands are covered by water they rise to the mouth of their burrow and protrude the syphons, but those that are situated so far in shore as to be uncovered at low-water, then plunge in to a depth of a foot or two. They never leave their burrows, except on compulsion, in the shape of the salt and spade of their enemy, the fisherman. The species, with a very straight margin to its shell, is the Pod Razor (S. siliqua) which attains a length of eight inches; that with a distinctly curved outline is the Sabre Razor (S. ensis).

A brief glance at some borers and excavators must suffice to close this long chapter. The small, upper figure in accompanying plate is the Red-nosed Borer (Saxicava rugosa), a species that largely helps the sea in its ceaseless attacks upon the coast line. It is the office of the Borer to excavate cells in the face of the rock, and as it is never solitary in its work, but attacks a rock in “gangs,” as a human excavator would put it, the result is the complete honey-combing of the surface. They may often be found free in crevices of the rocks and about the roots of seaweeds—that Alsatia for a very varied population. The shell has a distorted look about it, and the valves will not fit properly, the ends gaping to allow the foot and the syphons free play. It is very variable, however, and consequently has been a splendid subject for the variety-mongers and species-splitters, who have manufactured quite a long list of species and genera out of it. It changes a good deal at different periods of its life, and thus affords opportunities for careful descriptions made from isolated specimens utterly disagreeing with each other; therefore, the individuals described must belong to different species! In its early state the shell is symmetrical, and has two minute teeth in each valve; but before it has reached maturity it has lost its claim to be considered graceful or symmetrical, and has either worn its milk-teeth out or abandoned them as useless. The shell is covered with ridges and wrinkles, and it is by their help that it carves out its chamber in the rock. Sometimes on turning aside a curtain of weeds from a rock-face you will see a large number of crimson points, which, however, instantly disappear if they have been ever so lightly touched by the weeds. These are the ends of the borers’ syphons, protruded from their ventilation holes; they are united almost to their extremities, and present the appearance shown in our figure.

The Piddock, or Finger Pholas (Pholas dactylus), is a much larger species with some difference of structure. Its pure white shell, though thin and fragile, is covered in front with rasp-like ridges, and by means of it the chambers and tunnels of the rock are bored. Holding to the rock with the clear crystalline foot, the Pholas gives its shell a swing half way round in one direction, then a swing back, and so by alternate half-revolutions, the rasping of the shell gradually excavates a chamber sufficiently large to shelter it, communication with the outer world being maintained by the large syphons. So far there is no very great difference between the Pholas and the Saxicava; but the Pholas is peculiar in that it possesses neither ligament nor hinge, and in addition to the orthodox two valves, it has some additional ones. The hinge-plates are reflected back over the beaks, and a powerful muscle is attached thereto to keep the ordinary valves together. Above these, and to protect this muscle, are two short accessory valves, and a third, which is long, and extends back over the dorsal edges of the big valves. In other species of Pholas these arrangements give scope for variation.

And now it is time we gave some thought to the one-valved and valveless mollusks of the shore.