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.
[Illustration: Fig. 19. Hinge of _Cardita sinuata_.]
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).
[Illustration: Fig. 20. Hinge of _Cytherea crycina_.]
[Illustration: Fig. 21. Hinge of _Lutraria elliptica_]
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.
[Illustration: Fig. 22. _Pholas dactylus._]
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).
* * * * *
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.
[Illustration: Fig. 23. _Teredo navalis._]