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.
[Illustration: Fig. 17. Hinge-teeth of _Arca barbata_.]
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.
[Illustration: Fig. 18. _Trigonia margaritacea._]