Fig. 12. Cypræa oniscus.
4. The Solaridæ 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 Scalaridæ, "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 Cerithiadæ, 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 Turritellidæ, 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 Melaniadæ, and 9. The Paludinidæ, 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.
10. The Litorinidæ, or Periwinkles, need no word from us.
11. The Calyptræidæ comprise the "Bonnet-limpet," or Pileopsis, and "Cup-and-saucer-limpets" (Calyptræa). They may be described briefly as limpets with traces of a spire left. The genus Phorus, however, is spiral, and resembles a Trochus. They have been called "Carriers" from their strange habit of building any stray fragments of shell or stone into their house, thus rendering themselves almost indistinguishable from the ground on which they crawl.