Part 6

DIVISION a.--ASIPHONIDA--is next subdivided into--

1. The Ostreidae, or oysters, which are deservedly a distinct family in themselves.

2. The Anomiadae, comprising the multiform and curiously constructed Anomia, with the "Window-shells" (Placuna).

3. The Pectinidae, 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 L25!

4. The Aviculidae, 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 Mytilidae, or mussels, including the Lithodomus, or "Date-shell," which bores into corals and even hard limestone rocks.

6. The Arcadae, 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 Trigoniadae, 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 Unionidae, comprising the fresh-water mussels.

DIVISION b.--SIPHONIDA Integropallialia.

1. The Chamidae, represented by the reef-dwelling Chama.

2. The Tridacnidae, 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 Cardiadae, or cockles.

4. The Lucinidae, 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 Cycladidae, 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 Astartidae, a family of shells having very strongly developed teeth, and the surface of whose valves is often concentrically ribbed.

7. The Cyprinidae, 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 Veneridae. 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 Mactridae 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 Tellinidae 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 Solenidae, 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 Myacidae 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 Anatinidae have thin, often inequivalve pearly shells. The genus Pandora is the form most frequently met with in collections.

7. The Gastrochaenidae embraces two genera (Gastrochaena 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 Pholadidae 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).

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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 Terebratulidae are the most extensive family of this class.