7. The _Cypraeidae_, or Cowries (Fig. 12), which owe their high polish
to the size of the shell-secreting organ (mantle), whose edges meet
over the back of the shell, concealing it within its folds. With these
is classed the "China-shell" (_Ovulum_).
[Illustration: Fig. 12. Cypraea oniscus.]
The second group, or _Holostomata_, is divided into nineteen families,
beginning with--
1. The _Naticidae_, whose type, the genus _Natica_, is well known to
all shell-collectors through the common _Natica monilifera_ of our
coasts.
2. The _Cancellariadae_, in which the shells are cancellated or
cross-barred by a double series of lines running, one set with the
whorls, and the other across them.
3. The _Pyramidellidae_, which are high-spired, elongated, and slender
shells, with the exception of the genus _Stylina_, which lives
attached to the spines of sea-urchins or buried in living star-fishes
and corals. 4. The _Solaridae_ or "Staircase-shells," whose umbilicus
is so wide that, as you look down it, the projecting edges of the
whorls appear like a winding staircase. It is a very short-spired
shell.
5. The _Scalaridae_, "Wentle-traps" or "Ladder-shells," which may be
readily recognised from their white and lustrous appearance and the
strong rib-like markings of the periodic mouths that encircle the
whorls.
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.