How to Classify the Shells for the Cabinet
All the foregoing processes, except the naming of your specimens, are more or less mechanical, and are only the means to the end—a properly arranged collection. For, however well a collection may be mounted, it is practically useless if the different shells composing it be not properly classified. By classification is meant the bringing together those kinds that most resemble each other, first of all into large groups having special characteristics in common, and then by subdividing these into other smaller groups, and so on. Thus the animal kingdom is divided, first of all, into Sub-kingdoms, then each Sub-kingdom into so many Classes containing those which have further characteristics in common, the Classes into Orders, the Orders into Families, the Families into Genera, and these again into species or kinds.
The Mollusca, or soft-bodied animals, of whose protecting shells your collection consists, form a sub-kingdom, and are subdivided into four classes:—
Cephalopoda.
Gasteropoda.
Pteropoda.
Lamellibranchiata (or Conchifera).
And these again into Families, Genera, and Species.
The space at our disposal being limited, it is impossible to do more than furnish some general outlines of the different forms. For further details it will be necessary to refer to one of the larger works, a list of which will be found on the last page.
CLASS I.—The CEPHALOPODA (Head-footed) contains those mollusca that, like the common Octopus, have a number of feet (or arms) set round the mouth, and is divided into those having two gills. (Order I. Dibranchiata); and those with four (Order II. Tetrabranchiata). Order I. is again divided into: (a.) Those with eight feet like the Argonaut (or Paper-nautilus, Fig. 4), which fable has so long endowed with the power of sailing on the surface of the ocean, (it is even represented in one book as propelling itself through the air!) and the common Octopus. (b.) Those with ten feet, such as the Loligo (or Squid, Fig. 6), whose delicate internal shell so much resembles a pen in shape; the Cuttle-fish (Sepia, Figs. 5 & 7), whose so-called “bone” (once largely used as an ink eraser) is frequently found on our southern coasts; and the pretty little Spirula (Fig. 8).
The only representative of the four-gilled order now living is the well-known Pearly Nautilus; but in former times the Tetrabranchiata were extremely numerous, especially the Ammonites.
CLASS II.—GASTEROPODA (Belly-footed) comprises those mollusca which, like the common snail, creep on the under-surface of the body, and with one exception (Chiton, Fig. 20) their shells are univalve (i.e., composed of one piece). But before we go further, it may be well to point out the names given to different parts of a univalve shell. The aperture whence the animal issues is called the mouth, and its outer edge the lip; each turn of the shell is a whorl; the last and biggest, the body-whorl, the whorls, from the point at the top, or apex, down to the mouth form the spire; and the line where the whorls join each other is called the suture. The axis of the shell around which the whorls are coiled is sometimes open or hollow, and the shell is then said to be umbilicated (as in Fig. 3b); when closely coiled, a pillar of shell, or columella, is left (as in Fig. 9). Sometimes the corner of the mouth farthest from the spire and next the columella, is produced into a channel, the anterior canal (as in Fig. 9); whilst where the mouth meets the base of the spire there may be a kind of notch which is termed the posterior canal. Most Gasteropods are dextral, that is to say, the mouth is to the right of the axis as you look at it; a few, however, are sinistral, or wound to the left (like Physa); whilst reversed varieties of both kinds are met with.
Gasteropods of the first order have comb-like gills placed in advance of the heart, and are hence termed Prosobranchiata. They are divided into two groups: (a) Siphonostomata (Tube-mouthed), in which the animal has a long proboscis, and a tube, or siphon, from the breathing-chamber that passes along the anterior canal of the shell, which in this group is well developed. They have a horny operculum, or lid, with which to close the aperture. (b) Holostomata (or Whole-mouthed). In these the siphon is not so produced, and does not want to be protected; accordingly the mouth of the shell is entire, i.e. has no canal. The operculum is horny or shelly. The former (group a) includes several families:
1. Strombidæ, comprising shells, like the huge Strombus, or “Fountain-shell,” which is so often used to adorn the mantelpiece or rockery, and from which cameos are cut.
2. The Muricidæ, of which the Murex (an extraordinary form of this is the “Venus’ comb,” Murex tenuispina, Fig. 9), the Mitre-shells, and the Red-Whelks (Fusus) are examples.
3. The Buccinidæ, taking its name from its type, the Common Whelk (Buccinum undatum), and including such other forms as the Dog-Whelk (Nassa), the Purpura, the strange Magilus, and the lovely Harp-Shells and Olives (Fig. 10).
4. The Cassididæ, or “Helmet-Shells.” Cassis rufa, from West Africa, is noted as the best species of shell for cameo engraving; with it are classed the “Tun” (Dolium) and the great “Triton” (Triton tritonis), such as the sea-gods of mythology are represented blowing into by way of trumpet, and which are used by the Polynesian Islanders to this day instead of horns.
5. The Conidæ, whose type, the “Cone-shell” (Fig. 11), is at once distinctive and handsome, but which in the living state is covered by a dull yellowish-brown periostracum that has to be carefully removed before the full beauties of the shell are displayed.
6. The Volutidæ, embracing the Volutes and “Boat-shells” (Cymba).
7. The Cypræidæ, or Cowries (Fig. 12), which owe their high polish to the size of the shell-secreting organ (mantle), whose edges meet over the back of the shell, concealing it within its folds. With these is classed the “China-shell” (Ovulum).
The second group, or Holostomata, is divided into nineteen families, beginning with—
1. The Naticidæ, whose type, the genus Natica, is well known to all shell-collectors through the common Natica monilifera of our coasts.
2. The Cancellariadæ, in which the shells are cancellated or cross-barred by a double series of lines running, one set with the whorls, and the other across them.
3. The Pyramidellidæ, which are high-spired, elongated, and slender shells, with the exception of the genus Stylina, which lives attached to the spines of sea-urchins or buried in living star-fishes and corals.
4. The Solaridæ or “Staircase-shells,” whose umbilicus is so wide that, as you look down it, the projecting edges of the whorls appear like a winding staircase. It is a very short-spired shell.
5. The Scalaridæ, “Wentle-traps” or “Ladder-shells,” which may be readily recognised from their white and lustrous appearance and the strong rib-like markings of the periodic mouths that encircle the whorls.
6. The Cerithiadæ, or “Horn-shells,” which are very high-spired, and whose columella and anterior canal are produced in the form of an impudent little tail, the effect of which, however, in the genus Aporrhais, or “Spout-shells,” is taken away by the expanded and thickened lip.
7. In the next family, the Turritellidæ, or “Tower-shells,” the type Turritella is spiral; but in the allied form Vermetus, though the spire begins in the natural manner, it goes off into a twisted tube resembling somewhat an ill-made corkscrew. The mouth in this family is often nearly round.
8. The Melaniadæ, and 9. The Paludinidæ, are fresh-water shells. The former are turreted, and the latter conical or globular. Both are furnished with opercula, but the mouth in the first is more or less oval and frequently notched in front, while in the latter it is rounded and entire.
10. The Litorinidæ, or Periwinkles, need no word from us.
11. The Calyptræidæ comprise the “Bonnet-limpet,” or Pileopsis, and “Cup-and-saucer-limpets” (Calyptræa). They may be described briefly as limpets with traces of a spire left. The genus Phorus, however, is spiral, and resembles a Trochus. They have been called “Carriers” from their strange habit of building any stray fragments of shell or stone into their house, thus rendering themselves almost indistinguishable from the ground on which they crawl.
12. The Turbinidæ, or “Top-shells,” are next in order, and of these the great Turbo marmoreus is a well-known example, being prepared as an ornament for the whatnot or mantelpiece by removing the external layer of the shell in order to display the brilliant pearly nacre below. These mollusca close their mouths with a horny operculum, coated on its exterior by a thick layer of porcelain-like shelly matter. With them are classed the familiar Trochus and other closely allied genera.
13. The Haliotidæ offer in the representative genus Haliotis, or the “Ear-shell,” another familiar mantelpiece ornament.
14. The Ianthinidæ, or “Violet-snails,” that float about in mid-Atlantic upon the gulf-weed, and at certain seasons secrete a curious float or raft, to which their eggs are attached, are next in order, and are followed by—
15. The Fissurellidæ, or “Key-hole” and “Notched limpets,” whose name sufficiently describes them. To these succeed—
16. The Neritidæ, an unmistakable group of globular shells, having next to no spire and a very glossy exterior, generally ornamented with a great variety of spots and bands.
17. The Patellidæ, or true Limpets, are well known to every sea-side visitor: large species, as much as two inches across, are to be found on the coast of Devon, but these are pigmies compared with a South American variety which attains a foot in diameter.
18. The Dentaliadæ, represented by the genus Dentalium, or “Tooth-shell,” are simply slightly curved tubes, open at both ends and tapering from the mouth downwards, and cannot be mistaken.
19. Lastly, we have the Chitonidæ, whose single genus Chiton possesses shells differing from all other mollusca in being composed of eight plates overlapping each other, and in appearance reminding one of the wood-louse. This animal is not only like the limpet in form but also in habits, being found adhering to the rocks and stones at low-water.
Order II.—Pulmonifera. Contains the air-breathing Gasteropods, and to it consequently belong all the terrestrial mollusca, though some few aquatic genera are also included. The members of this order have an air-chamber instead of gills, and are divided into two groups, (a) those without an operculum, and (b) those having an operculum. Foremost in the first group stands the great family—
1. Helicidæ, named after its chief representative, the genus Helix. It also includes the “Glass-shell” (Vitrina), the “Amber-shell” (Succinea), and such genera as Bulimus, Achatina, Pupa, Clausilia (Fig. 13), etc., which differ from the typical Helix in appearance, possessing as they do comparatively high spires.
2. The Limacidæ, or “slugs,” follow next; of these only one, the genus Testacella, has an external shell stuck on the end of its tail; the rest have either a more or less imperfect shell concealed underneath the mantle, or else none at all.
3. The Oncidiadæ are slug-like, and devoid of shell.
4. The Limnæidæ embrace the “Pond-snails,” chief of whom is the well-known, high-spired Limnæa stagnalis. Other shells of this family associated with Limnæa are, however, very different in shape; for instance, Physa has its whorls turning to the left instead of to the right; Ancylus (Fig. 24), or the freshwater limpet, is of course limpet-like; while Planorbis, or the “Coil-shell,” is wound like a watch-spring.
5. The Auriculidæ includes both spiral shells, such as Auricula and Charychium, and a limpet-like one Siphonaria.
At the head of group b stands 1, Cyclostomidæ. Cyclostoma elegans is a common shell on our chalk-downs, and well illustrates its family, in which the mouth is nearly circular. Foreign examples of this genus are much esteemed by collectors. The other two families are, (2) Helicinidæ and (3) Aciculidæ.
Order III.—Opisthobranchiata. These animals carry their gills exposed on the back and sides, towards the rear of the body. Only a few have any shell. 1. The Tornatellidæ, which have a stout little spiral shell. 2. The Bullidæ, in which the spire is concealed (Fig. 14). 3. The Aplysiadæ, where the shell is flat and oblong or triangular in shape. The remaining families are slug-like and shell-less.
Order IV.—Nucleobranchiata. Derives its name from the fact that the animals constituting it have their respiratory and digestive organs in a sort of nucleus on the posterior part of the back, and covered by a minute shell. As they are pelagic, the shells are not readily to be obtained. They are divided into two families, Firolidæ and Atlantidæ.
CLASS III.—PTEROPODA. Like the last, these pretty little mollusca are ocean-swimmers. The members of one division of them, to which the Cleodora belongs, is furnished with iridescent external shells.
CLASS IV.—The LAMELLIBRANCHIATA (Plate-gilled), or Conchifera (Shell-bearing), includes the mollusca commonly known as “bivalves,” the animal being snugly hidden between two more or less closely fitting shelly valves. The oysters, cockles, etc., are examples of this class. The two valves are fastened together near their points, or beaks (technically called umbones), by a tough elastic ligament, sometimes supplemented by an internal cartilage. If this be severed and the valves parted, it will be found that in most cases they are further articulated by projecting ridges or points called the teeth, which, when the valves are together, interlock and form a hinge; the margin of the shell on which the teeth and ligament are situated is termed the hinge-line. A bivalve is said to be equivalve when the two shells composing it are of the same size, inequivalve when they are not. If the umbones are in the middle, the shell is equilateral (Fig. 15); but inequilateral when they are nearer one side than the other (Fig. 16). If the shell be an oyster or a scallop, you will find on the inside a single circular scar-like mark near the centre; this is the point to which the muscles that close the valves and hold them so tightly together are attached. In the majority of bivalves, however, there are two such muscular impressions, or scars, one on either side of each valve of the shell. The former group on this account are often called Monomyaria (having one shell-muscle), and the latter Dimyaria (having two shell-muscles). In the last named the two muscular impressions are united by a fine groove (or pallial-line), which in some runs parallel to the margin of the shell (Fig. 15), whilst in others it makes a bend in (pallial-sinus) on one side of the valve towards the centre (Fig. 16). In Monomyaria it will be found running parallel to the margin of the shell. It marks the line of attachment of the mantle or shell-secreting organ of the animal to the shell which grows by the addition of fresh matter along its edges, so that the concentric curved markings so often seen on the exterior correspond in their origin with the periodic mouths of the Gasteropods. The bivalves are all aquatic, and many bury themselves in the sand or mud by means of a fleshy, muscular foot. These are furnished with two siphons, or fleshy tubes, sometimes united, sometimes separate, through which they respire, drawing the water in through one and expelling it by the other. Those kinds whose habit it is to bury themselves below the surface of the mud or sand are furnished with long retractile siphons, and to admit of their withdrawal into the shell, the mantle is at this point attached farther back, giving rise to the pallial-sinus above described; this sinus is deeper as the siphons are proportionately longer, and in many cases, too, the valves do not meet at this point when the shell is closed.
Attention to these particulars is necessary when arranging your bivalves, as on them their classification depends, the class being divided into—
a. Asiphonida (Siphonless).
b. Siphonida Integro-pallialia (with Siphons).—Pallial-line entire.
c. Siphonida Sinu-pallialia (with Siphons).—Sinus in pallial-line.
Division a.—Asiphonida—is next subdivided into—
1. The Ostreidæ, or oysters, which are deservedly a distinct family in themselves.
2. The Anomiadæ, comprising the multiform and curiously constructed Anomia, with the “Window-shells” (Placuna).
3. The Pectinidæ, taking its name from the genus Pecten, or “Scallop-shells,” of which one kind (P. maximus) is frequently to be seen at the fishmongers’ shops. The “Thorney oysters” (Spondylus) take rank here, and are highly esteemed by collectors, one specimen indeed having been valued at £25!
4. The Aviculidæ, or “Wing-shells,” among which are numbered the “Pearl-oyster” of commerce (Meleagrina margaritifera). The strange T-shaped “Hammer oyster” belongs to this family, as does also the Pinna. The Pinnas, like the mussels and some other bivalves, moor themselves to rocks by means of a number of threads spun by the foot of the mollusc, and termed the byssus, which in this genus is finer, more silky, than in any other, and has been woven into articles of dress.
5. The Mytilidæ, or mussels, including the Lithodomus, or “Date-shell,” which bores into corals and even hard limestone rocks.
6. The Arcadæ, or “Noah’s-ark-shells,” characterized by their long straight hinge-line set with numerous very fine teeth (Fig. 17). The “Nut-shell” (Nucula) belongs to this family.
7. The Trigoniadæ, whose single living genus, the handsome Trigonia (Fig. 18), is confined to the Australian coast-line, whereas in times now long past they had a world-wide distribution.
8. The Unionidæ, comprising the fresh-water mussels.
Division b.—Siphonida Integropallialia.
1. The Chamidæ, represented by the reef-dwelling Chama.
2. The Tridacnidæ, whose sole genus Tridacna contains the largest specimen of the whole class of bivalves, the shells sometimes measuring two feet and more across.
3. The Cardiadæ, or cockles.
4. The Lucinidæ, in which the valves are nearly circular, and as a rule not very attractive in appearance, though the “Basket-shell” (Corbis) has an elegantly sculptured exterior.
5. The Cycladidæ, whose typical genus Cyclas, with its round form and thin horny shell, is to be found in most of our ponds and streams.
6. The Astartidæ, a family of shells having very strongly developed teeth, and the surface of whose valves is often concentrically ribbed.
7. The Cyprinidæ, which have very solid oval or elongated shells and conspicuous teeth (Fig. 19). The “Heart-cockle” (Isocardia) belongs to this family.
Division c.—Siphonida Sinu-pallialia.
1. The Veneridæ. The hard, solid shells of this family are for elegance of form and beauty of colour amongst the most attractive a collector can posses. Their shells are more or less oval and have three teeth in each valve (Fig. 20).
2. The Mactridæ are somewhat triangular in shape, and may be at once recognised by the pit for the hinge-ligament, which also assumes that form, as seen in the accompanying figure of Lutraria elliptica (Fig. 21).
3. The Tellinidæ comprise some of the most delicately tinted, both externally and internally, of all shells. In some, coloured bands radiate from the umbones, and well bear out the fanciful name of “Sunset shells” bestowed upon them. Their valves are generally much compressed.
4. The Solenidæ, or “Razor-shells,” rank next, and are readily recognised by the extreme length of the valves in proportion to their width, and also by their gaping at both ends.
5. The Myacidæ or “Gapers,” have the siphonal ends wide apart (in the genus Mya both ends gape), and are further characterized by the triangular process for the cartilage, which projects into the interior of the shell. One valve (the left) is generally smaller than the other.
6. The Anatinidæ have thin, often inequivalve pearly shells. The genus Pandora is the form most frequently met with in collections.
7. The Gastrochænidæ embraces two genera (Gastrochæna and Saxicava) of boring mollusca, which perforate shells and rocks, and also, the remarkable tube-like “Watering-pot-shell” (Aspergillum) which is hardly recognisable as a bivalve at all.
8. The Pholadidæ concludes the list of bivalves, and comprises the common rock-boring Pholas (Fig. 22) of our coasts and the wood-boring shipworm “Teredo” (Fig. 23).
Although the Brachiopoda, or “Lamp-shells,” are not true mollusca, they are not very far removed from them, and are so often to be found in cabinets that it will not do to pass them over, especially since in past times they were very abundant, an enormous number occurring in the fossil state. Only eight genera are now living. Shells belonging to this class are readily recognised by the fact of one valve being larger than the other, and possessing a distinct peak, the apex of which is perforated. The Terebratulidæ are the most extensive family of this class.