Part 5

6. The Cerithiadae, 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 Turritellidae, 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 Melaniadae, and 9. The Paludinidae, 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 Litorinidae, or Periwinkles, need no word from us.

11. The Calyptraeidae comprise the "Bonnet-limpet," or Pileopsis, and "Cup-and-saucer-limpets" (Calyptraea). 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 Turbinidae, 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 Haliotidae offer in the representative genus Haliotis, or the "Ear-shell," another familiar mantelpiece ornament.

14. The Ianthinidae, 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 Fissurellidae, or "Key-hole" and "Notched limpets," whose name sufficiently describes them. To these succeed--

16. The Neritidae, 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 Patellidae, 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 Dentaliadae, 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 Chitonidae, 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. Helicidae, 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 Limacidae, 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 Oncidiadae are slug-like, and devoid of shell.

4. The Limnaeidae embrace the "Pond-snails," chief of whom is the well-known, high-spired Limnaea stagnalis. Other shells of this family associated with Limnaea 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 Auriculidae includes both spiral shells, such as Auricula and Charychium, and a limpet-like one Siphonaria.

At the head of group b stands 1, Cyclostomidae. 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) Helicinidae and (3) Aciculidae.

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 Tornatellidae, which have a stout little spiral shell. 2. The Bullidae, in which the spire is concealed (Fig. 26). 3. The Aplysiadae, 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, Firolidae and Atlantidae.

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.