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.