P. æquisulcatus. The Southern species above referred to as resembling P. irradians or dislocatus. It has about twenty stout, well-rounded ribs, with concentric wrinkles between them. The "ears" are equal. The size and shape are as in P. irradians. The coloring is whitish, varying to shades of brown, and excentrically marked with spots or waving transverse lines of chestnut or horn.
With the pectens we leave behind all pelecypods with the true filamentous type of gill. In the order Eulamellibranchiata we find all the families to be possessed of the basketwork or crisscross branchial structure. Each gill is fundamentally formed of filaments which project downward, and then suddenly turn upon themselves and ascend; the ascending and descending limbs are connected, however, by processes of presumably vascular function, and adjacent filaments are connected by truly vascular channels. The closer connection of the filaments to form of each gill a sort of plate is further effected [pg439] by connective tissue, which sometimes is dense enough (though always porous) to obscure the reticulated appearance of the organ. Furthermore, the edge of the outer lamella of the outer gill unites with the mantle; likewise the edge of the inner lamella of the inner gill unites with the body-mass, and behind the body-mass with the gills upon the other side of the animal. This union of the four gills in a straight median line behind the foot and body-mass forms a septum, which divides off from the mantle cavity a "suprabranchial" cavity. Into this cavity the excreta are discharged, and with it the anal or excurrent siphon connects. The branchial or incurrent siphon thus pours its current of water into the lower mantle cavity, to bathe the gills and feed the mouth, while the fresh water is not contaminated by any open and wide connection with the excretory processes. In many of the forms where the siphons are very long and united, this septum, formed by the union of the dorsal edges of the inner gills back of the foot, is continued far into the siphons, constituting the division of the two into the anal and branchial siphons.
Another feature of this order is the marked tendency to mantle fusion. As there are always siphons, the mantle-edges must necessarily have fused at two points at least, in order to have formed the siphons. The tendency, however, is for the fusion to continue, narrowing the pedal opening more and more, as though the mantle were endeavoring to envelop the entire animal in a bag or sac open only at one end (siphonal opening), but yet slightly ripped along one seam (pedal opening).
This order includes the great majority of pelecypods, and admits of many very confusing modifications in its most essential features.
In this, the first family of the order, most of the characteristic features just spoken of are present. Mantle fusion, however, has not progressed beyond a degree which is just sufficient to form the siphons. There is a byssiferous gland. The name Carditidæ refers to the heart-shape of the shells. [pg440]
Genus Cardita
C. borealis. A very common species north of Cape Ann, reported also in deeper water as far south as Hatteras. It is a solid, obliquely heart-shaped shell, with beaks elevated and turned forward. It has about eighteen to twenty rounded radiating ribs, which are broader than the grooves between them. The epidermis is rusty-brown. The hinge is strong, with two prominent teeth; the margins of the shell are strongly crenulated within. Length one inch; height about the same. This is not strictly a between-the-tides species, but its valves are often cast upon the beaches. Fine specimens may be obtained by dredging at Bar Harbor or Eastport.
C. floridana. A very common Floridian species. Its seventeen or eighteen ribs are very heavy, and are rendered nodulous by the numerous bluntly raised scales upon them. The shell is lengthened, and has a well-defined lunule and an external ligament. In color it is yellowish-white, with purple markings when young. Length one inch; height three fourths of an inch. It lives in sheltered bays; detached valves are often found cast upon the beach.
Genus Astarte
Notwithstanding their rather small size, there is a trimness about the shells of Astarte which, taken together with their rich chestnut epidermis, makes them very interesting and desirable additions to the cabinet. The family is one of cold-water range, with a few representatives on the New England coast. Live specimens are not easily captured, for they live in deeper water than one would care to wade in, nevertheless they are frequently cast upon the beaches after storms. The animal has a very short efferent siphon, large labial palps, and very obliquely placed gills. Not more than twenty living species of Astarte are known, and all of them belong in, or are wanderers from, polar seas. Fully three hundred fossil species of this genus have been described from Mesozoic rocks in all parts of the world. The question naturally arises whether in these its dying days Astarte has sought colder waters in which to pass its remaining existence, or whether climates have changed. The fact that such strictly [pg441] tropical genera as Voluta and Conus are found fossil in northern Siberia seems to be evidence in proof of great climatic variations.
A. castanea. This species has a thick and heavy shell for one so small, with round ventral margins and prominent beaks. The surface is very slightly undulated and is covered with a thick chestnut epidermis with pale and dark zones. The hinge is broad and flat, with a prominent cardinal tooth upon the right valve, which accurately fits into a pit formed by two teeth on the left valve. Length one inch; height one inch.
A. undata. A species slightly larger than the last, with ten to twenty prominent concentric undulations.
The identification of the many species of Astarte often becomes exceedingly difficult on account of the fact that the genus is a remarkably uniform one in its main characters and is infinitely variable in its minor details.
Genus Arctica
This family is represented by one species upon the New England coast, which is much sought after by collectors. It is one of the larger clams, but is too uncommon to have received a popular name and to be generally well known by the fishermen along the shore.
A. (Cyprina) islandica. This clam has a thick though not ponderous shell, with prominent beaks and a stout, prominent ligament. Its cardinal teeth are large, but its laterals are very small. A striking feature of this species, which at once separates [pg442] it from all other shells of our coast of similar size, is the coarse, wrinkled epidermis of black to chestnut color. The shell is very suggestive of a large, overgrown Astarte. Length three and a half inches; height three inches. It is found of all sizes, down to one fourth of an inch in diameter, upon muddy stations in moderately deep water near the mouths of rivers; but the surest way to get it is to search the beach after a hard onshore gale.
This family finds its metropolis in warmer waters, and the best representatives of it must be sought for upon the Floridian beaches or in the shallow bays of that sandy coast. A long vermiform foot is a characteristic of the animal, as is also the fact that at times there is only a single gill upon each side of the body. In some of the Lucinidæ, more so than in most other pelecypods, the chief function of the gills is to furnish a brooding-place for the thousands of ova which the creatures generate. Sometimes a Lucina will be captured with the gills swollen out of all proportion, and literally stuffed with tiny, microscopic eggs. At such times the gills lose all semblance of branchial organs. The shells are orbicular, with depressed small umbones, a distinct lunule, a semi-external ligament, two cardinal teeth, and laterals (a variable feature). There is no pallial sinus. The color is white, and the outlines are rounded.
Genus Lucina
L. tigrina. The largest of the group, measuring three inches across and nearly the same in height. It is flatly convex and radially ribbed by a great number of costæ which are crossed and decussated throughout with concentric ridges. The color is white. This fine shell lives only in southern Florida and is abundant in shallow water on sandy stations. (Plate LXXX.)
L. floridana. One of the most abundant bivalves in Florida, often cast in thousands upon the beaches. It lives in shallow protected waters and upon sand-flats which are partly exposed at low tide. It has the usual round outlines of the genus, with exceedingly small umbones, directed forward, and a smooth surface, save for fine growth-lines of a light straw-color. The shell is pure white. Diameter about one inch. (Plate LXXX.)
L. pennsylvanica. A species often associated with the last, but about twice as large. Its thin but tough epidermis clings to the shell in raised concentric lines, giving it a circularly ribbed appearance. The most prominent feature of this species is a depressed line upon either valve extending obliquely from the umbones to the posterior ventral margin, [pg443] causing, at the margin, a pucker or break in its even continuity. The lunule is large and ribbed; the teeth are small; the color is pure white, with a light straw-tinted epidermis. (Plate LXXX.)
L. dentata. In this species the shells are thin and white, with well-marked concentric lines crossed by deep oblique narrow furrows bent at nearly right angles to the lines of growth and forming teeth around the margin. Found along the entire coast.
L. californica. The best-known California species of this genus. It is pure white, with fine concentric lines, and varies in size from one half of an inch to one and a half inches in diameter. The lateral teeth are the stronger, and the lunule is upon the right valve only. With these exceptions this species preserves the usual characters of this genus.
L. nuttallii, belonging to the southern shores of California, is decussated like fine wickerwork. The shell is flattened and ridged along the hinge-margin. The color is white. Diameter about one inch in large specimens. (Plate LXXX.)
Genus Loripes
L. edentula. A species which is seldom captured alive. Its home is in the open sea, but vast quantities of its valves are occasionally thrown upon the beaches south of Hatteras, as far as the Gulf of Mexico. On account of the weak hinge ligament, the valves become easily separated, and it is not always easy to find two that will exactly match. Loripes preserves the same circular outlines as Lucina, but is more ventricose, and the hinge and teeth are very feeble. This species is a little over two inches in length and slightly under two inches in height. It is pure white without, and is finely striated with growth-lines; bright orange within, especially about the pallial line and muscle-scars. (Plate LXXX.)
Genus Tellina
If we should create an aristocracy of beauty among the bivalves, as has been done by conchologists among the gasteropods, this family would deserve high rank. Most of the American species of Tellina are too small to impress one very greatly with their beauty, but in the West Indies and in the tropical Pacific waters are some wonderfully handsome shells belonging to this or to the allied genus Macoma. However, we have in Florida the very striking T. radiata, a truly beautiful shell, which, were it less common, would be highly prized in collections. Rarity, no doubt, adds a wonderful luster to shells as well as to gems. The [pg444] animal of Tellina has long, slender siphons, which are separate. The gills are small, and the outer lamella or branchial fold is directed dorsally. Although the foot shows evidences of a byssogenous gland, there is no actual byssus. The shells are porcelanous and translucent, equivalve, with an external ligament, and at least two cardinal teeth in each valve.
T. radiata. This species varies from three to four inches in length, and from one and a half to one and three fourths inches in height. It is highly polished, shining white, and yellowish about the umbonal region, and has three wide rays of bright pink extending from the beaks to the ventral margin. On account of its remarkable coloration it is called the "setting-sun shell." Its surface is smooth. A common variety is of a uniform creamy-white color with carnation beaks. This shell is always offered for sale by the marine curio- and shell-dealers in Florida seaside resorts, and at Atlantic City, Cape May, and elsewhere.
T. alternata resembles the last in shape, being flattish and oblong, but it is more pointed or angulated posteriorly. There is also a slight twist in the posterior end of the shell. Its surface is decorated by numerous impressed concentric lines. The color is uniformly white or yellowish, with pink rays. Length two and a half to three inches; height two and a half inches. Found from Hatteras to the Gulf of Mexico, in shallow, sandy stations and on beaches after storms. (Plate LXXXI.)
T. bodegensis. The most striking species of Tellina found on the west coast, and of rather northern range. It is about two inches in length, creamy-white in color, and ornamented with very fine concentric lines. A very characteristic feature of the genus is present in this species as in the last, namely, a slight twist or bend to one side in the posterior end of the shell.
T. tenera. A little Tellina found in New England waters, which, though very small, is worthy of mention on account of its abundance upon all our beaches north of Hatteras. It is only about half an inch long, and slightly over one quarter of an inch high. It may be distinguished by its general generic characters, its iridescent surface, and its delicately marked lines of growth; sometimes it is rose-tinted. Its remarkably long siphons are shown in the accompanying figure.
Genus Macoma
M. secta. A very pretty thin glossy shell from two to three inches in length and about one fourth less in height, which occurs from Monterey southward. The posterior end of the shell is contracted and slightly bent; the pallial sinus is deep. There is an external ligament. (Plate LXXXI.)
| PLATE LXXX. | |
|---|---|
| 1, Lucina tigrina. | 3, Lucina pennsylvanica. |
| 2, Lucina floridana. | 4, Lucina nuttallii. |
| 5, Loripes edentula. | |
| PLATE LXXXI. | |
|---|---|
| 1, Tellina alternata. | 3, Donax variabilis. |
| 2, Macoma secta. | 4, Donax californicus. |
| 5, Ræta canaliculata. | |
M. nasuta. Another Californian species, with a less shining surface and a very thin light-brownish epidermis. The twisting of the posterior [pg445] margin is striking. It has very fine cardinal teeth. Length two to three inches.
M. baltica, M. proximo, M. tenta. The first two of these species are dingy in color, and are covered with a dirty-looking, thin epidermis. The first has a wide range to Norway and Scotland, and is exceedingly common all along our coast from Maine to Georgia. It occurs plentifully in the Hudson River above New York, and in all sandy or muddy bays. It is rounded in outline, while the next species (M. proxima) is more pointed posteriorly. When these species are buried in the sand their two long, slender siphons project above, one supplying the animal with a constant current of fresh water, and the other ejecting that which, having passed the gills and labial palps, is exhausted of its air and nutriment and contaminated by waste products. M. tenta is white, with very fine lines of growth, and polished and shining within. It is warped posteriorly. Length three fifths of an inch; height two fifths of an inch.
Genus Donax
The Donacidæ are represented upon our coasts by comparatively few species, all belonging to the genus Donax. The shells are under an inch in length, and have a peculiarly shortened anterior portion; thus they are inequilateral. Their ventral margins are finely denticulate. The pallial sinus is deep. There are two or three cardinal teeth and a variable number of laterals.
D. variabilis. This little shell, the common Floridian form, exhibits every imaginable scheme of color-combination, and defies general or comprehensive description. Probably the most usual pattern developed is a bluish-white background with purplish radiating lines. Another is a pure-white foundation with red lines. The surface is striated longitudinally with excessively fine riblets. Length one half of an inch or slightly more. In March and April these shells are thrown alive in millions upon the Florida beaches. Each wave seems to be laden with them, and when the foaming waters of each spent breaker recede, the little shells lie still upon the sand for a moment, glittering like jewels in the sunlight; then, with a sudden protrusion of the foot and a quick turn, they all disappear like a flash, buried in the sand before the next breaker strikes them. One must be very quick to catch these active little mollusks. (Plate LXXXI.)
D. californicus. Professor Keep very aptly describes this shell, which is very common in California, as "short and stumpy, cut nearly square off at one end (anterior), and tapering to a rounded edge at the other." The same description fits both this and the Floridian D. variabilis so well that the two forms may be one and the same species. [pg446] If they were shells of arctic range it would be easier to connect them; but as neither is a cold-water form, it becomes more difficult to place them under one name. The increasing number of similar species being found upon the Atlantic and Pacific sides of Central America points almost conclusively to the existence of a waterway between the two oceans, somewhere between North and South America, at no very remote geological period. Possibly the closing of some such waterway through the isthmus connecting the oceans has separated these forms geographically, thus leading us to separate them specifically. (Plate LXXXI.)
This is a very large family of universal distribution and with a great number of genera and subgenera, rather confusedly gathered into subfamilies. The dominant genus Mactra comprises the largest bivalve upon the east coast of the United States.
Genus Mactra
The mactras live in the sand near the margin of the water, often upon the exposed open coast. The mantle is open, except where it is fused to form the siphons. These are short and united. The outer gill is dorsally directed; that is to say, it does not depend into the mantle cavity as does the inner gill, but projects just the other way; it is also smaller than the inner one. The foot is strong, bent, and tongue-shaped, and no doubt well adapted to the rough life in the surf and the heavy, shifting sands. The main characteristic of the mactra shell is the prominent triangular-shaped fossette, or cartilage plate, situated internally and just under the beaks. The shells are equivalve, and devoid of bright colors or striking sculptural features. Internally the pallial line is plain, and the sinus well marked but not deep.
M. solidissima is one of the very commonest, if not the commonest large bivalve of the New England, Long Island, and New Jersey [pg447] beaches. It is covered with a thin brownish or straw-colored epidermis, usually more or less worn away in adult specimens and wholly absent from the dead valves found upon the beaches. There is a fragile V-shaped cardinal tooth, which is generally broken away in forcing open the valves; the lateral teeth are long and thin, and striated on their receiving surfaces. The length of this "giant clam" is from four to seven inches. This is the first shell the collector will find (north of Hatteras) when he goes to an open, sandy beach.
M. similis. A small variety of the species last described, of similar outline, but less heavy and strong. Its range is from Hatteras southward to Brazil. Passing from New Jersey to Hatteras, Mactra solidissima diminishes in size, and after passing that faunal barrier the Northern form is entirely replaced by this Southern variety. The shell is brighter, though not exactly bright-shining. Its habits are entirely similar to those of the Northern form. On the west coast of Florida it is the most abundant bivalve upon the beaches.
M. ovalis. A species of Northern range, which so greatly resembles M. solidissima as to suggest identity; but the lateral teeth are smooth and not striated, while the shell itself seldom attains a length of over four inches.
M. lateralis. An exceedingly common shell, found in all the salt-marshes and tidal estuaries of our coast. It is much smaller than the mactras just described, being not over one half of an inch in length. It is white, and is covered with a dirty brown epidermis; the hinge is strong, with deep cartilage fossette and a V-shaped marginal tooth. The laterals are long and slender. The animal has a strong foot and two slender, long siphons which are yellow in color.
Genus Ræta
R. (Labiosa) canaliculata. In this species the cartilage fossette is the same as in Mactra; the marginal teeth are feeble, and there are no laterals. The shells of Mactra which we have examined are heavy or at least solid, but Ræta is thin and fragile; it is pure white in color and is gracefully ornamented by concentric raised ribs. The shell is inflated and gapes slightly posteriorly; it is shining white within and is deeply grooved concentrically in accordance with the external ribs. Length two and a half, height two inches. From Hatteras to Mexico, this is one of the commoner beach shells, but since, as in some other species already named, the hinge ligament and teeth are feeble, the valves become separated by the buffeting of the waves, and are cast disjointed and singly upon the beaches. (Plate LXXXI.)
In many respects this is the most distinguished pelecypod family. It is the largest in number of genera and species, is of world-wide distribution, and supplies to hungry man a number [pg448] of comestible species. Several of its genera have been named after various goddesses and nymphs on account of the great beauty of their shells. But from a purely biological point of view also the Veneridæ have claims to distinction, for they seem to represent about the highest type of shell-development among the bivalves. The test is extremely hard and often porcelanous. It is solid without being ponderous and unwieldy in the sense of causing the animal to be helplessly overburdened by the sheer weight of its shell. The valves are equal and have three stout cardinal teeth. The foot is strong, the animal is active, the siphons are free or partially united, and the gills are more or less crumpled and folded. There is a compactness and well-regulated arrangement of the organs in the Veneridæ, which impress one who has examined the anatomy of many pelecypods much as a well-built, efficient, and space-economizing machine would impress a mechanic. None of the organs are weak or crowded out to furnish room for the undue or awkward development of other organs. A good balance is preserved between them all, the result being a high type of pelecypod.
If we do not expect to find any of the more remarkable forms of the Veneridæ upon our shores we shall be the less disappointed when returning from collecting-expeditions. Nevertheless, there are a few species of Cytherea, Dosinia, and Tivela in North American waters which are very handsome shells; but those of our species which are included under the generic name Venus are in point of beauty not worthy of their promising name.
Genus Venus
V. mercenaria. This is the common "hard-shell clam," the "Little Neck" of the hotels and restaurants, with which every one in New York and New England is familiar. The species ranges from Cape Cod to Florida, and occupies two different kinds of station: one in shallow muddy bays near low-tide mark, in estuaries, etc.; the other the sandy stations in open deeper bays or the open ocean. Those coming from "outside" have thinner, more delicate shells, with thin, concentric ribs. They burrow in the sand or mud, or crawl quite rapidly. The siphons are short and are united almost to their ends. The mantle is open (save for the siphons), and the edges are delicately frilled. In the United States this species is very extensively gathered for the markets. When the oyster season closes in April, the hard-shell clam comes into vogue, the season lasting until September or October. The clams are [pg449] extensively gathered in the estuaries from Cape May to Cape Cod. Men armed with rakes drift about in small boats and fish the sluggish creatures up out of the mud. They will live for many days out of water if kept in a cool place. Smaller or immature clams are better for the table, since they are not so tough as their elders. A few features of the shell of this species are noticeable—the rather prominent umbones directed forward, the heart-shaped lunule, the external ligament, the concentric sculpturing of growth-lines, the pure-white interior (with occasionally violet zones about the muscle-scars and margin), the three stout cardinal teeth, the sharp-angled pallial sinus, and the ventral margin finely crenulated within. The greatest length is about three inches and the height two and a half inches, but these dimensions exceed those of the clams usually served upon the half-shell. V. mercenaria ranges into Florida, increasing in size and bulk as it meets the warmer waters of the Gulf Stream. The Floridian form has received the varietal name of mortoni. This massive clam sometimes weighs five pounds, and is fully six inches in diameter. Aside from its great size and thickness, the characters of the shell are wholly those of the smaller Northern form. It is abundant in shallow, muddy, brackish bays, and is gathered by men who "tread out" the clams, feeling for them in the mud and sand with bare feet. The flesh is tough and disagreeably strong, yet the natives appear to relish it, especially when it is made into a chowder.
V. cancellata. The most abundant species of Venus in Florida, particularly near Tampa and on the west side. It preserves all the characteristics of the genus, and is decorated with a series of high, narrow, concentric ridges, between which are much smaller and more numerous radiating riblets. The color is dirty white to light yellow-brown without, and white and violet within. There are three cardinal teeth and no laterals. The lunule is heart-shaped; the posterior margin is broadened into a wide, concave area into which the concentric ridges do not pass. Length one to one and a fourth inches; height the same. Sometimes the beaches in Florida will in places be banked up with the valves of this very abundant Venus. As they live in shallow water upon sandy stations, they are easily dislodged by the heavy storms which sometimes sweep that usually placid coast. (Plate LXXXII.) [pg450]
Genus Cytherea
C. (Callista) gigantea. One of the most beautiful Floridian shells. It is nearly or quite six inches long and about three and a fourth inches in height. The shell is porcelanous, is covered with a pinkish, livid epidermis, and is decorated with numerous longitudinal color-rays of lilac. The rays are usually broken and interrupted. Within it is china-white, and has a deep pallial sinus. It is very abundant on the west coast of Florida, at Sanibel Island, Egmont Key, Lake Worth, and on beaches. (Plate LXXXII.)
C. maculata. This species is more oval in outline than the last. It is of a fawn-color, blotched or waved with violet-brown, and is porcelanous and shining. The epidermis is also shining and corneous. Within it is china-white in color. The pallial sinus is very deep and wide. Length two and a half to three inches; height one and a half to two inches. This also is a shell to be looked for upon the beach after storms, but it will probably not be seen on the east coast of Florida, although it occurs on the Keys and also at Hatteras. Both of the species have been used as food, but not very extensively. The flesh is not lacking in tenderness, but it has a peppery taste. (Plate LXXXII.)
Genus Dosinia
There are two species of this very graceful genus of Veneridæ south of Hatteras, D. discus and D. elegans. These dosinias have united siphons. The foot is very large and strong, and the mantle margin is both papillaceous and plicated. The shells of these two species are about three or three and a half inches in length and of the same height, and are very flat. The ventral margin is almost a perfect circle, and the beaks are small and pointed. The lunule is small and cordate; the valves are very gracefully sculptured in fine, regular, impressed, concentric striæ, which are finer and less elevated in D. discus than in D. elegans; the shell is porcelanous and white, and is covered by a straw-colored, very fine, transparent epidermis. Within, the strong cardinal teeth are placed upon a long fossette. There is a deep pallial sinus. D. discus is characteristic of the east coast of Florida, while D. elegans is only found upon the west side of the State. Both may be looked for upon the beach, though living specimens must be captured with the dredge. (Plate LXXXII.)
| PLATE LXXXII. | |
|---|---|
| Venus cancellata. | Cytherea (Callista) gigantea. |
| Cytherea (Callista) maculata. | Dosinia discus. |
Genus Tivela
The coast of California is also favored by the Veneridæ, a number of large species belonging to several genera being present.
| PLATE LXXXIII. | |
|---|---|
| Tivela crassatelloides, one half natural size. | Tapes laciniata, one half natural size. |
| Chione succincta. | Chione simillima. |
T. crassatelloides. Perhaps the largest and finest of the California species. It is so common in its between-tides station that it used to be collected by plowing a furrow through the sand, the big shells being thrown out upon either side like potatoes. The shells attain a length of five and a half inches, and are porcelanous, thick, and heavy. The margins of the valves are also thick and rounded. The exterior is perfectly smooth, yellowish-white in color, and decorated [pg451] with purplish rays arranged in various groupings; over all is a glossy epidermis. The hinge-teeth are very strong, and a prominent anterior lateral tooth on the left valve is noticeable. There is an external ligament; the pallial sinus is well marked, but not very deep. (Plate LXXXIII.)
Genus Chione
There are three species on the west coast. This genus is very closely allied to Venus, and is considered by many to be only a subgenus of the latter. All three species are white, solid, heavy shells which exhibit the same scheme of sculpturing, namely, numerous longitudinal ribs crossed by larger and less frequent concentric ridges.
C. simillima is shaped almost exactly like the east-coast Venus cancellata, but its concentric ridges are very numerous and sharp. There is a strongly marked cordate lunule, and hardly any pallial sinus. Length one and a half to two inches. (Plate LXXXIII.)
C. succincta has fewer concentric ridges and has a large lunule. There are purple zones within and touching the heavy cardinal teeth. Length two and a half inches. (Plate LXXXIII.)
C. fluctifraga has broad but flatter or well-rounded (not sharp) concentric ridges, no lunule, a small ligament, a fairly deep pallial sinus, and a finely crenulated margin (within) all around the valves, except when interrupted by the hinge ligament. The shell is solid and shining, and marked by deep-purple zones within. Length one to two inches. Found in southern California.
Genus Tapes
This is a prominent genus of the Veneridæ. Its chief feature is the total want of lateral teeth. There are several large species in California.
T. staminea, T. laciniata. These are the most noticeable species. The former is sold in the San Francisco markets as the "hard-shell clam." The shell of this species is from two to three inches in length and of somewhat less height, rounded, and heavy. The sculpturing consists of radiating ribs traversed by strongly marked growth-lines. The teeth are strong, and the pallial sinus is deep. T. laciniata is much like the last, but its surface is reticulated by a crisscross design that is very striking. It also has a deep pallial sinus. These two species are of Southern range. They burrow in gravelly places, among stones, between tide-marks, and their presence may frequently be detected when walking along the beach at low tide by seeing a sudden jet of water ejected from a little hole in the mud or gravel. The animal, becoming alarmed at the approach of something, suddenly snaps shut its valves, and thus forces out some water through its siphons. The foot is very strong and often byssiferous in this genus. (Plate LXXXIII.) [pg452]
Genus Saxidomus
S. nuttallii. The last of the larger west-coast Veneridæ which we shall examine. The genus is very closely allied to Tapes, and no doubt might better be considered as its subgenus. The shell is brownish-white, three to four inches in length, and heavy, rough, and coarse-looking, with irregular concentric ridges. There is no lunule, but a large external ligament is very prominent just back of the beaks. Within it is white, with a deep pallial sinus. Just below the ligament area is a zone of translucent, agate-like shell-structure. This large clam, with several others of the same genus, lives in shallow water near shore.
Genus Petricola
P. pholadiformis. Along the Jersey coast, especially about Atlantic City, there are small patches of clay or hard tenacious mud which lie just at the edge of the sea. As the beating of the surf gradually encroaches upon these hard fragments of once extensive clay-beds, many specimens of this curious bivalve mollusk are washed out from their burrows in this clay and cast upon the sandy beach. The animal has long siphons which are united about one third of the way to their ends. Although this creature is allied to the Veneridæ, the degree of mantle fusion present is much greater, leaving in this case only a fairly long slit through which the foot may protrude. The shell is very dissimilar to that of any of the Veneridæ, and in many respects, judging from a conventional standard, is a remarkable form. It is exceedingly thin, is of a chalky texture, gapes widely posteriorly, and is ornamented with transverse ribs, which are feeble upon the long produced posterior portion of the shell, but strong in the anterior part, where they are crossed by the coarse lines of growth, which appear like vaulted scales upon the ribs. The color is a dull white. There are two cardinals in each valve and no laterals. Length one and a quarter to two and a half inches; height one half to three quarters of an inch. This species passes its entire life in the burrow it has made for itself in the clay. Its only motions are made in climbing to the entrance of its burrow or in retreating far out of sight within its depths. Its siphons are tipped with a dark or drab color as a measure of protection from the predaceous crabs which range about the mud surfaces near tidal marks, and are always upon the lookout for succulent bits of flesh.
P. carditoides. A petricola of the Californian coast, very similar in its habits to the east-coast form. It bores, however, not only into clay, but also into soft rocks in order to effect a permanent lodgment. Often this species occupies a hole which it has discovered already existing; in this case its shell grows to fit the surroundings. The hinge-teeth, and sculpturing are frequently reduced and sometimes are quite obsolete; but the shell has the chalky, thin texture that is characteristic of the genus. [pg453]
There is another family of rock- and mud-boring bivalves which superficially resemble the petricolas, but their anatomical organization removes them to a little distance from the Petricolidæ. They are considered further on, under the name of Pholadidæ.
Genus Cardium
This is another important family of the Pelecypoda, which, with the Veneridæ, the Mactridæ, and the Tellinidæ, is entitled to distinction on account of its high organization, its high type of shell, its universal geographical distribution, and its great beauty. The dominant genus is Cardium—a name given on account of the fact that the shell is shaped like a heart. The animal has a very long, recurved, and surprisingly strong foot. The mantle-edges are quite freely open to the siphonal region and are highly papillate and waved. The siphons are usually very short, but have wide exterior openings and are also strongly papillate about their margins. The gills are obliquely situated, and seem to be thicker and heavier than is usual in pelecypods; they are much folded, the outer lamella being the smaller. With such a pointed foot and such short siphons Cardium is obviously not a deep-burrowing genus; but it does partially bury itself in soft semi-liquid sand. Its progression is effected by leaps which it is enabled to make by reaching far out with its foot, then suddenly twisting to one side and throwing the shell several inches. The shell is ventricose, is closed or gapes slightly posteriorly, and has prominent subcentral umbones and an external ligament. The sculpture-scheme of Cardium (if the shell is not perfectly smooth) is one of radiating ribs or ridges, which, being regular, form a perfect crenulation of the ventral margin. There are two cardinal teeth in the center, and one lateral tooth upon both the anterior and the posterior side: the latter are situated at some little distance from the beaks.