The first includes the simpler type of gill as represented in the accompanying figure (A). Its filaments are short and not reflected. The second, represented in Fig. B, has the filaments long, reflected, and connected (each filament to its adjacent ones) by means of surface cilia. The third type of gill resembles the last except that the ends of the filaments of the outer gill are attached to the mantle, and the ends of the filaments of the inner gill are attached to the foot or visceral mass. The fourth type of gill is far more highly [pg421] developed. The adjacent filaments are connected, not by cilia, but by vascular channels, and the depending and recurved portions of each filament are also connected in a manner presumed to admit of the passage of blood. This gives to the organ the appearance of basketwork or network. The ends of the filaments are attached to the mantle and to the visceral mass, as in the last order, forming cavities or chambers above (see Fig. C). This order includes the great majority of the Pelecypoda.
The fifth and last order is confined to but two rather obscure families, in which the gill-development is carried to the point of substitution of muscular partitions which form a separate chamber connected with the mantle cavity by a narrow slit, the surface of the chamber having respiratory functions (Fig. D).
The collector of living marine objects who has become fascinated by their beauty and who derives pleasure from examining the greater beauty of their structures will find a rich field for research in the pelecypod gills. But unless he has time at his disposal and some skill with a microscope, he would do well not to venture too far into this alluring but difficult and vexatious subject.
Our coastal waters are fairly rich in bivalves, but the most strikingly handsome species of this class—and there are many of them—are inhabitants of warmer seas. Of the great number of bivalves to be found upon our own shores we can only mention briefly some of the commonest species.
Family NUCULIDÆ
In this family the gills are of the simplest and most primitive type. The mantle-edges are entirely open along the ventral margin, but they are united posteriorly in some of the Nuculidæ to form two short siphons. The palps are exceedingly large. The foot is also large, and by its disk-like surface is adapted to creeping, much as is a gasteropod foot; its edges are often crenulated or scalloped, and there is no trace of a byssus. The shells are [pg422] equivalve, and their most prominent feature is the hinge, which is beset with sharp, comb-like teeth in two straight rows, one upon each side of a central pit, a space reserved for the ligament.
Genus Nucula
N. proxima. The commonest of several species of the genus found along the New England coast. It is oblique, with a light-olive epidermis, nacreous interior, and finely crenulated margins. Its length is about one quarter of an inch. The animal has no siphons. It lives in either muddy or pebbly stations near the shore, and probably exists in countless thousands in all the bays and harbors of the New England coast.
Genus Leda
L. tenuisulcata. This species has a longer shell than the preceding; it is more produced behind, and is narrowed into a blunt, slightly gaping point. The epidermis is light greenish, and the shell has a pearly luster within. The outer surface of the shell is concentrically grooved; the beaks are smooth. The foot is disk-shaped, and the animal is provided with small siphons. The length of this shell is about one inch. Its station is the muddy bottom in shoal water on the New England coast.
Genus Yoldia
Y. limatula. This species has the same sort of toothed hinge exhibited in the two preceding species, but the shell is considerably larger (two inches), with a smooth greenish glazed epidermis. The beaks are nearly central. The interior of the shell is light bluish and pearly. The animal has two slender, short, united siphons and a disk-shaped foot with simple margins. This species can make excellent use of its foot, for it crawls with rapidity, and also can execute leaps in an astonishing manner. Found in muddy stations in shoal water on the New England coast.
Y. thraciæformis. A much larger species, found on the New England coast. It may be distinguished by a rib-like wave extending obliquely from the umbones to the ventral margin of the shell. It measures over [pg423] two and a half inches in length and one and a half inches in height, and is dark olive-green. The foot is like that of Y. limatula. The labial palps are exceedingly large.
Y. sapotilla. The shell is ovate, prolonged on one side, thin, fragile, translucent, and covered with a thin, glossy epidermis, greenish in color. The anterior half is semi-oval, the posterior portion narrowed and compressed; within it is pearly-white, with a triangular cartilage cavity, and sixteen to eighteen pointed teeth on each side. In length it is little less than an inch. Found from Long Island Sound northward.
Genus Solenomya
Two species of this genus, which are greatly prized by collectors on account of the extraordinary development of the epidermis of the shell, occur in New England waters. The strong corneous periostracum of a deep chestnut-color projects considerably beyond the margins of the valves; the shells are exceedingly thin and fragile, and are marked with radiating lines. The two species are easily distinguished by their difference in size. S. velum is about one inch long and one half of an inch high. S. borealis, of arctic range, is quite twice that size. Their anatomy is similar in essentials to that of Yoldia. These species live near shore in sandy bottoms, and are occasionally found upon the beach between tides. Chelsea Beach, just north of Boston, is an often-quoted locality.
There are several families of this order which are well represented in both the Atlantic and the Pacific waters of the United States. The characteristic feature of this group of bivalves is the filamentous gill, that is, a gill with the filaments long, doubled back, and united to each other only by ciliary junctures. These [pg424] junctures are so slight that they break readily when a specimen of a gill is handled, leaving the filaments free. The presence of a byssal gland in the foot, and often a well-developed byssus, is another characteristic of this order.
Genus Anomia
This is a family of peculiar and highly specialized forms. Anomia has an irregularly rounded shell, with one convex and one flat or concave valve. There is no regular hinge or well-defined hinge-margin, but a raised fossette, or cartilage plate, occupies a position at the top of the valves. In the flat valve there is a large oblong hole just under the apex, through which projects a calcified byssus, by means of which the animal secures itself to oysters, dead shells, stones, or any solid object. The anomias, which have become stationary in habit, have practically lost their foot. The gills are very large and curved, while all the organs seem to be abnormally placed on account of the huge byssus and byssal muscle. For the byssus to pass, as it does, through a specially prepared hole in one of the valves is an extraordinary departure from the conventional types of byssiferous species. The heart is not traversed by the intestine. Altogether, then, Anomia is a very curious genus.
A. simplex. The commoner large form of New England. It varies from one to three inches in diameter, is exceedingly irregular in shape, and its surface is variously undulated and plaited in accordance with the surface of the object to which it is attached. Thousands of these valves, disjointed and separated, are cast upon the beaches all along our Atlantic coast. They are light green to salmon- or copper-color, generally fragile and scaly, and have a peculiarly dulled (as though greased) nacre. [pg425]
A. aculeata is much smaller than the last species, and the surface of the convex valve is roughened by prickly scales. The smaller valve is very thin, with an almost circular aperture for the byssus. Its diameter is about half an inch. Found attached to stones, etc., about the holdfasts of Fucus.
A. lampe, a California species ranging more to the south, is small (half an inch), and greatly resembles the east-coast A. aculeata, save as regards the smooth surface of the former.
Genus Placunanomia
P. macrochisma. This species occurs on the Californian coast. It has the same sort of calcified byssus as Anomia, and also the hole to accommodate it in the smaller valve. The scar formed by the muscle which controls the byssus is curiously rayed in this genus. The normal shape of the shell is circular; it is pearly within, and of a greenish tinge without. Two to four inches in diameter. (Plate LXXVIII.)
Genus Arca
The Arcidæ may always be distinguished by the long row of comb-like teeth upon the hinge, the total want of siphons (a fact which may be discovered in the shell alone by its lack of a pallial sinus), and the solid trapezoidal or round shell, with its tendency to strong ribs radiating from the umbones. The periostracum, or epidermis, is heavy and often velvety or even hairy. The animal of Arca has the gills placed in an oblique position; the foot is large and strongly developed. No trace of mantle fusion exists. The mantle-edges bear a row of composite eyes. The gill-filaments are entirely free, there being even no ciliary junctions.
A. pexata. An exceedingly common species in Long Island Sound and on the coast of New Jersey. The shell is oblong, with prominent beaks directed forward. The hinge-teeth are arranged in a row posterior to the beaks, while just under the beaks are a number of irregular cartilage-pits. About thirty-two to thirty-six radiating ribs ornament the shell. The inner margins of the shell are rather deeply scalloped; the epidermis is thick, shaggy, and dark [pg426] brown. Length two to two and a quarter inches; height two inches. A. pexata is one of the few pelecypods which have red blood, a fact which accounts for its popular name of "bloody clam."
A. ponderosa. The most prominent Arca upon our Atlantic coast, especially south of Virginia. The beaks of this shell are very prominent, and do not approach each other very closely on account of the wide dorsal margin of the shells, the large space between the beaks being occupied by an external ligament. The hinge-teeth are smaller in the middle. There are from twenty-five to twenty-eight ribs; the ventral margin of the valves is somewhat contracted in the middle. Length two and a half inches; height two inches. Very heavy and solid. This exceedingly common species is often cast upon the beaches south of Hatteras in numbers beyond computation. In life it is covered with a heavy, coarse, velvety epidermis, almost jet-black in color. (Plate LXXVIII.)
A. noæ. The well-known "Noah's-ark" shell, a common species along the shores of the Southeastern States. It also occurs in the Mediterranean. Unlike most arcas, it spins a byssus, by means of which it attaches itself to the under surfaces of stones at low water. The hinge-margin is perfectly straight and regularly toothed. The beaks are high and are situated very far forward. The dorsal margin of the valves is strikingly large, and is marked by lattice-like grooves; it forms a concave surface over an inch in width between the umbones. The shell is strongly ribbed without. The ventral margin of the valves is sinuous and gapes slightly at a central point for the accommodation of the byssus. (Plate LXXVIII.)
Genera Mytilus, Modiola
The Mytilidæ are a large family, including the mussels, so commonly found between tides upon all shores. The two ordinary genera Mytilus and Modiola, which often occur associated together upon our eastern coast, may readily be distinguished one from the other by the position of their beaks. In the former genus the beak is terminal, that is, it is situated at the very tip-end of the shell; in the latter genus it is not quite terminal. Aside from this simple feature, the genera are practically the same, the animals being identical. Mytilus and Modiola, commonly known as mussels, have acquired the stationary habit and are provided with a relatively small foot (in some species amounting to no more than a mere tubercle); but in place of a useful foot is a well-developed byssogenous gland, which secretes an [pg427] exceedingly tough, fibrous byssus. By means of this the animal may not only attach itself firmly to any sort of object, but may actually move about. Fibers of the byssus are thrown out, and, as though possessed of life, they seize upon objects in the direction in which Mytilus desires to move, and by contraction of the muscles which control the byssus the animal is drawn forward, the operation being then repeated. It must be a very tedious method of travel. However, the Mytilidæ are not restless creatures. They pass their lives content to remain firmly fixed to some object, and their movements, if any, are within a very narrow circle. The animal has very long gills of the usual filamentous type. The mantle fuses at one point to form an anal siphon, the lower or branchial one being, therefore, functional. The palps are large. One curious feature of this group is the enormous extent of the genital glands, which actually appear to pervade the entire structure of the animal, gills, mantle, foot, and all. At times the whole being is a veritable receptacle for myriads of tiny yellowish eggs.
The Mytilidæ are of universal distribution, and are for the most part of littoral or shallow-water range. Some species are esteemed as an article of food, notably Mytilus edulis.
Mussel-culture has been profitably carried on in France for a long time, but the genus has not been utilized to any extent as food in this country. [pg428]
Mytilus edulis is exceedingly abundant along the New England coast. It lives in colonies, often between tides, in pebbly stations among large rocks. It is always so fastened by its byssus that the beaks of the valves point downward, and the ventral margin projects above the sand or mud. The color of the epidermis in the adult form is black or a deep blue-black; the shell proper is violet. Hinge-teeth are wanting. Length two and a half inches. A variety of this species, M. pellucidus, is often found associated with the typical form. It is brightly rayed in green and yellow. M. edulis occurs in Europe, where it is quite extensively used as food. It also occurs (by importation) in San Francisco Bay.
M. hamatus has a more southern range than has M. edulis, and is found from the Chesapeake southward. Its shell is considerably twisted just below the umbonal region, while its whole surface is densely striated. Its color is dark fuscous. Length one to two inches. It is found in great numbers attached to oysters by its byssus. In Florida it is very common. (Plate LXXVIII.)
M. californicus. A common west-coast species, which finds its station among rocks exposed to the surf. It is about the same size as M. edulis, but has a lighter brownish glossy epidermis and several conspicuous, well-rounded, radiating ribs. The animal is orange-colored.
Modiola modiolus. After violent storms upon the New England coast, when a large Laminaria is wrenched from its rocky bed and washed ashore, its roots will, in many cases, be found clinging to this large mussel. The thick glossy epidermis of deep chestnut-color folds over the margin of the valves, and the outer surface, especially near the ventral margin, is covered with a tough hairy growth. Length four to five inches. The animal is orange-red. The great size and swelling fullness of this species at once distinguish it from all others upon our coast. It is not a shallow-water form, but is extremely common upon all beaches north of Hatteras. It also occurs in Alaskan waters to Puget Sound, where it is known as the "great horse-mussel." Sometimes these large modiolas may be found in rocky tide-pools which are washed by the surf at high tide. They hide far back in the crevices of boulders, and [pg429] are not easily detected. They are not uncommon in such stations at Mount Desert, Maine.
M. plicatula. This species of Modiola ranges from Canada to Hatteras, and inhabits the tidal waters of streams, sheltered muddy reaches among reeds, and tidal flats. It is a dingy-looking shell, ornamented with numerous radiating ribs, which are very fine upon the anterior portion of the shell. The epidermis is thin and of a dirty yellowish-green color. Within it is silvery-white. In the ventral margin there is an opening for the accommodation of a byssus. Length about three inches in New England; slightly larger at Norfolk. There is probably no muddy reach of land that is exposed at low tide from Maine to the Carolinas that is not inhabited by M. plicatula.
M. tulipa. This is the "show shell" of the genus Modiola in American waters. It belongs to the Southern fauna, and, like the others of its genus, it moors itself to wood or other solid objects by means of its byssus. It is of about the same size as the last species, but is perfectly smooth and of a bright-yellowish color, with dark rays over the central portion.
M. recta. A species of Modiola fairly abundant on the Pacific coast from Vancouver to southern California. It has a long, narrow shell with a dark-brown glossy epidermis; lighter posteriorly, and with a short hairy growth. The interior is white. Length three to four inches.
This third order of Pelecypoda is made up of a number of families in which the gills are filamentous, but in which the filaments [pg430] have become united at their ends to the mantle (on the outer gills) and to the body-mass (on the inner gills). Here also the first traces of the higher type of gill found in the next order appear; that is, a gill in which the filaments are connected by vascular channels instead of by cilia. This order therefore is presumed to connect the true filamentous type of gill with the fully developed lamellar type. As the filaments do not always fuse with the mantle or foot, the description of the pseudolamellibranch gill becomes somewhat vague, and there is some room for doubt whether this order should exist at all.
Most of the genera placed in it are monomyarian; that is, they have but one adductor muscle, or if two, the anterior one is very small and unimportant. The mantle-edges are entirely open, there being no regularly formed siphons. As a rule, the foot is exceedingly small or quite aborted.
In this family there is an exception to the rule just stated: the foot is long. There is a byssus, and the gills (by their outer ascending filaments) are concrescent with the mantle.
The interior of all the Aviculidæ is of a highly polished nacre, and some of the larger species have been extensively collected for mother-of-pearl. To the Aviculidæ belong the pearl-oysters (Meleagrina margaritifera).
A delightful air of romance and adventure surrounds the pearl-fisheries, about which much has been written. The pearl is an abnormal product formed by the deposition of nacre around some foreign object, such as a grain of sand, or—usually, it is said—an egg which has failed to develop properly. They are seldom found in individuals having favorable conditions of growth, but those which are greatly crowded together and covered with various parasites and growths are most productive of them. The pearl is the same in substance as the nacreous shell of the pearl-oyster, and its iridescence is due to the fine striæ of the undulating layers of which it is composed. The pearl as well as the shell is produced by the shell-glands of the mantle. The value of the [pg431] pearl is based on the brilliancy of the nacre, the size, and the form. Luster, or "orient," is to the pearl what brilliancy is to the diamond. The round form has the greatest value, next the pear-shape, and lastly the oval. Pearls of inferior value are produced by other species than Meleagrina margaritifera, the true pearl-oyster. Many bivalves secrete similar bodies, but as they have the nature of the shell, those produced by mollusks having lusterless interiors are of no value. Beautiful pink pearls are found in the gasteropod Strombus gigas, the conch-shell of the West Indies. The shells of the pearl-oyster are themselves very valuable, being the "mother-of-pearl" used in the manufacture of buttons, knife-handles, inlay-work, and so on. They have three grades of commercial value; of the first are the "silver-lipped," from the South Seas, of the second the "black-lipped," from Manila and Ceylon, and of the third the "bullock-shells," from Panama.
Genus Avicula
A. atlantica. A Floridian species which is often found attached by its byssus to large algæ which have been washed upon the beach. The shell is reddish-brown, with many white radial lines. Its obliquely oval shape, the long wing-like extension of the dorsal margin, the inequivalve shells, and the byssal sinus placed just below the forward extension of the hinge, are characteristics which will enable one to recognize the species at once. The structure of the shell is unusual in being so largely composed of organic matter that it possesses elasticity near its thinner edges.
Genus Pinna
P. muricata and P. seminuda. Two species of this family which are very abundant in Floridian waters and are often to be found associated upon the muddy or sandy shores of bays. Like Modiola plicatula, they attach themselves by means of their byssus to shells or stones which lie under the surface. With the pointed end of the shell directed downward, they remain almost wholly concealed in the sand. Only about one quarter of an inch of the broad end of the shell projects above the sand. The edges of the valves are as sharp as knife-blades, and great caution must be used while walking barefooted along a beach inhabited by Pinna. The shells of both species are of a triangular wedge-shape and are composed of a translucent crystalline matter. P. muricata is beset with triangular erect scales which project out like spines. Their edges are also sharp like pieces of broken glass. P. seminuda also has these raised scales, but they are far more numerous [pg432] and much smaller, and are arranged in closely packed radiating rows, the scales becoming obsolete near the umbones. The mantle of the animal is doubly fringed, and much decorated upon that portion which may be protruded from the shell. The posterior adductor is very large and strong; the anterior one has almost disappeared. The foot is long and slender, but is evidently a useless appendage, for the animal's habit is stationary. The byssus is large and very strong. Gloves have been made from the byssal fibers of Pinna mixed with silk. The gills are very long and are distinctly not filamentous, the adjacent filaments being clearly united by vascular channels. A very curious development of the osphradium (see page 339) into an erectile process capable of projection beyond the margin of the shell is a unique feature of Pinna. Sometimes thousands of pinnas are torn loose by storms and cast upon the beaches. They do not range north of Hatteras. (Plate LXXIX.)
Genus Perna
P. ephippium. A West Indian shell, also quite common on the east coast of Florida, and especially upon the Keys. It attaches itself by the byssus in great bunches to mangrove roots, sticks, and all manner of rubbish in brackish water. It has a very flat, irregularly shaped shell of a horn-color, and is from two to three inches across. The distinguishing feature of Perna is the hinge. It is straight and without teeth, but is marked by a row of transverse grooves to which the ligament is attached. These grooves constitute a row of cartilage-pits.
This family probably interests more people than any other one in the class save Aviculidæ. The latter furnishes the pearl-oysters, the former the edible oysters, the two thus appealing to two very strong human cravings—those of vanity and hunger.
Few realize what an enormous business the oyster trade has become in the United States. The value of it is stated to be over thirteen million dollars annually, twenty-five million bushels of oysters being taken from the Chesapeake alone. The edibility of the oyster has been known from early times, for vast heaps of empty oyster-shells, known as kitchen-middens, occur in various parts of the world. Some of them are of such size and extent as to warrant the belief that their formation must have required centuries. Shell-mounds are found along the coasts of Florida and are of some archæological value. The cultivation of oysters as recorded by Pliny dates from the first century B.C., at which [pg433] time a Roman named Sergius Orata first cultivated oysters at Baiæ. The acuteness of the gastronomic sense of a Roman epicure is thus described by Juvenal:
He could tell
At the first mouthful if his oysters fed
On the Rutupian or the Lucrine bed
Or at Circeii.
The poet Gay's opinion is expressed thus:
The man had sure a palate cover'd o'er
With brass or steel, that on the rocky shore
First broke the oozy oyster's pearly coat,
And risk'd the living morsel down his throat.
The methods employed in oyster-farming resemble those of agriculture, in that the bed is prepared, seed is sown, superfluous and foreign growths are weeded out, enemies are driven off, and the crop is harvested at stated seasons. The oyster is ovoviviparous; that is, it retains its eggs until they are partly matured. These are held in the gills and mantle folds until the time of spawning, which begins in May and lasts through the summer months. The larvæ are ejected as ciliated spheres, called spat, and swim freely about for some time, often several days, before finding a resting-spot. The oyster-grower secures many of the larvæ by placing in their way substances to which they can attach themselves. The American culturist strews his carefully prepared beds with empty oyster-shells, on which the spat settle, and the seed is thus secured; for the spat, once fastened, lose the power of locomotion and become fixed. At the end of a year the shells which hold the young oysters (now about an inch long and called "fry") are taken up, and the fry are thinned out and replanted, or are sold to other oyster-farmers.
During the period of their growth the oysters are sometimes transplanted several times. At the end of three to five years they have attained marketable size, and the beds are then harvested and prepared for another crop. Some oystermen have several acres of bottom under cultivation. These areas are obtained by purchase or grant from the State, and their limits are as defined [pg434] as are the fenced-off acres of upland meadows. The business of the oyster-culturist is to plant the young oysters and watch their development, keeping the beds thinned, that the oysters may not be too crowded for their normal and symmetrical growth, and protecting them from their enemies, of which there are many. The principal enemies of the oyster are the starfish and the predaceous mollusks Urosalpinx and Nassa. Whole beds have been known to be destroyed in a single night by the visitations of starfishes, hence a constant watchfulness is required on the part of the oysterman. Policing the oyster-farms is another of his cares, for pirates abound, and a bed may be robbed in the night as easily as an orchard may be despoiled of its fruit. Oyster-culture is carried on extensively in Long Island Sound, on the coasts of New Jersey and Virginia, and in the Chesapeake Bay. The oysters from certain localities are esteemed more than others, the flavor of the oyster being very dependent upon the purity of the water and on the organisms upon which it feeds. It has been definitely shown that oysters grown in contaminated waters have been the agents of transmitting disease, notably typhoid fever and cholera.
Oyster-culture has reached the highest degree of perfection in France, where miles of muddy shores left by the ebb-tide are utilized for the purpose. The famous parc at Arcachon, covering ten thousand acres, illustrates the system generally practised. The spat is there collected on tiles coated with cement, piled in layers crosswise, and secured in crates. These tiles are exposed in favorable localities and collect the swimming embryos. The rough cement holding the spat is then chipped off the tiles and placed in wire trays. These ambulances, or caisses ostreophiles, placed between short posts, protect the fry from their enemies. At low tide the young oysters are carefully examined and sorted, and at the right age they are removed to open areas. The parc is divided into regular squares by canals of suitable width, some of the squares having banks and gates to retain the water of the receding tide if desired. Each man's parc is separated from that of his neighbor by canals of greater width, and often by stakes having a few branches on the ends. These [pg435] pignons, which sway in the wind, are said to frighten away the predaceous ray which is apt to hover about the preserves. The parcs are finally thinned out by sending the oysters to other parcs to be fattened. There is a celebrated parc d'élevage at Marennes. It is a collection of artificial ponds, the floors of which are covered with algæ, which harbor vast numbers of diatoms and other microscopic organisms on which the oysters feed. The green diatom (Navicula ostrearia) gives to the oysters of Marennes the green color and peculiar flavor which is so much esteemed by the epicures of France. To the American, however, the green oyster is not acceptable.
Genus Ostrea
O. virginica. A description of this species, our common oyster, is hardly necessary. Every one has seen the rough, shaggy, unlovely shell. The hinge is toothless, but has a wide depression for the ligament. The animal, having stationary habits, has practically no foot at all. There is but one large adductor muscle, around which curve the gills, the latter being united to each other posteriorly. The mantle margin is finely and doubly fringed. Although Ostrea is a stationary mollusk, it has no byssus.
O. virginica has been introduced at San Francisco, where it lives well, but does not seem to multiply very rapidly. The native species, O. lurida, is about two inches long, dark in color, and stained a purplish hue. It is not very delicately flavored.
O. frons. This species has a thinner shell than O. virginica, with coarsely serrated margins. It occurs in beds in the neighborhood of mangroves all along our South Atlantic shores. It cannot compare with its Northern relative in flavor, but, like the European Ostrea edulis, it is sometimes "not bad."
Genus Pecten
The scallop-shells (Pecten) are objects too familiar to require any general description. The rounded valve, usually ornamented with radiating ribs, and the wing-like projections (called "ears"), from each side of the umbonal region, are never-failing characters. The outline of Pecten has been considerably employed in conventional designs for mural decorations; indeed, the figure [pg436] of a well-known Mediterranean pecten (P. jacobius), found commonly in Palestine, became an emblem of religious significance during the middle ages. Returning crusaders fastened to their garments a specimen of "St. James's shell" as an evidence of the fact that they had been to the Holy Land, and the design of the shell came to be adopted upon many coats of arms and also in the insignia of various orders of devout and adventurous knights of the middle ages.
The animal is of the highest type of monomyarian mollusks, that is, of bivalve mollusks with only a single adductor muscle. Unlike most pelecypods, which have a very small foot, Pecten rarely has a byssus, and is neither a stationary nor a sluggish creature. It can propel itself through the water by spasmodically closing and opening its valves, in an eccentric, darting sort of flight, though most of the time it rests quietly upon the bottom. The mantle is entirely open and highly ornate about its margin, which is, furthermore, the seat of many eyes, capable, apparently, of no mean degree of vision. The adductor muscle is very large and strong, and occupies a central position, about which the gills circle; the latter are plainly filamentous.
P. magellanicus. The largest of the east-coast species of Pecten. It is a Northern species, and was long known by the name of P. tenuiscostatus—a name given to it on account of its very numerous radiating striæ; but it was later discovered that the North Atlantic form was in reality the same as the P. magellanicus of Patagonia. The latter being the older name and entitled to priority, our shell became P. magellanicus, the other name falling within its synonymy. The length and height of this scallop are from five to five and a half inches. One valve is more convex and slightly larger than the other, the smaller being lighter in color. The valves gape considerably along their upper margin below the hinge. The cartilage-pit is deep. North of Cape Ann this large species is of common occurrence in moderately deep water. The deeper bays and arms of the sea which everywhere penetrate the Maine coast are its favorite resorts. A good way to catch pectens is to lower a fishing-line at a spot where the fishermen report "scallop-ground," and drag it along over the bottom. Sooner or later it will enter the open shell of some pecten, which will instantly close its valves upon the string and allow itself to be drawn out of the water. In Maine these large scallops are eaten, but they have not found great favor in the city markets. In color they vary from reddish through brown to ashen.
P. islandicus. A species not so large as the last, with more prominently raised ribs (about fifty to one hundred in number), which are [pg437] covered with a multitude of erect scales. The ribs persist upon the interior of the shell. The "ears" are unequal in size. The color is light orange to reddish-brown, with zones of darker and richer color. One valve is flatter than the other and more lightly colored. Greatest length about three inches; height three and a half inches. This is a Northern shell, belonging to the waters of Newfoundland, but its valves are frequently cast upon the New England beaches.
P. irradians. The common shallow-water species of the Atlantic coast. This exceedingly abundant species does not range north of Boston, but finds an extensive habitat to the south. Its metropolis is Cape Cod, Long Island Sound, and the Jersey coast waters. This is the true scallop of the Boston and New York markets. Only the adductor muscle is eaten. In this species there is not so marked a difference in the convexity of the two valves. There are about twenty elevated and rounded ribs, the spaces between them being also rounded. The color is variable, from a blackish horn to ashen gray. The "ears" are equal; the interior is shining, and grooved to correspond with the external ribs. Length two and three fifths inches, height two and a half inches. On clear, calm days the immature individuals of this species may often be seen in shallow water disporting themselves most gaily, skipping about and snapping their valves in great glee. Their mantle margins are very ornate in scarlet or orange, with rows of bright-blue eyes. Care must be taken in handling scallops, especially the large P. magellanicus, for a finger caught between the valves will be badly bruised.
P. dislocatus. South of Hatteras this species takes the place of its Northern relative, P. irradians. It is somewhat more globular and perhaps not quite so large, but the resemblance between the two species is very marked. It has from twenty to twenty-two rounded ribs, with very numerous concentric wrinkles between them. In color it is white, tinged with purple or yellow, with occasionally a system of blood-red lines crossing the ribs transversely. The diameter is about one and a half inches. This species has the same playful habits as P. irradians, and is not at all easy to capture. When pursued it dives into the mud, and sets up such a commotion by the spasmodic opening and shutting of its valves that the water at once becomes roiled, and the scallop is safely hidden in its murky depths. [pg438]
P. hastatus. There are several very beautiful pectens on the Pacific coast of the United States, and two of these—one of northern and the other of southern range—resemble very closely the two Atlantic species of relatively north and south range, P. islandicus and P. irradians or dislocatus. The first is P. hastatus of Puget Sound. Valves of this exquisite shell are even occasionally found in California. It has very much the aspect of P. islandicus—unequal ears and an ovate form. The most curious thing about this shell is the fact that a different system of sculpturing exists upon the two valves. Upon one the ribs are even and the whole surface is covered with raised scales; upon the other the ribs are alternately of large and small size. Color rose-pink, with deeper bands; one valve lighter-colored, suffused with delicate pink. Height two and a half inches. (Plate LXXIX.)