Fig. 246. Cerithium giganteum (Lamarck).
Cerithium is a marine shell, which is found in muddy bottoms, on ships, and more frequently at the mouths of rivers, but rarely beyond the point to which the tide reaches. The genus is numerous in species. Such are Cerithium fasciatum (Fig. 244) and Cerithium aluco (Fig. 245).
The Giant Cerithium, Cerithium giganteum (Fig. 246), is the living analogue of a magnificent fossil species belonging to the tertiary formation. The single known example of this species belongs to the Delessert Museum at Paris. A manuscript note by Lamarck, attached to this specimen, relates that this shell was first brought to Dunkirk in 1810 by an Englishman, one of the crew of an English ship. The English sailor had drawn it up from the bottom of the sea with the sounding-lead from a bed of rocks off the coast of Australia.
The fourteenth family, Pyramidellidæ, contains Chemnitzia and Pyramidella, extremely pointed shells.
The fifteenth family, Naticidæ, contains Lamellaria and Natica; the last of which is found in most seas.
The second section of the Prosobranchiata is termed Siphonostomata, which are characterized by a spiral imperforate shell, the animal of which has sometimes a horny operculum, and is furnished with an elastic trunk, the margin of the mantle acting as a siphon. They are carnivorous.
The first family is the Cypræidæ, containing the well-known Cypræa and Ovulum.
The Cowries, or Cypræa, are brilliant, smooth, and polished, oval-shaped, or oblong convex, with edges rolling inwards and longitudinal openings, narrow, arched, dentate on both edges, and notched at the extremities. The spiral, placed quite posteriorly, is very small, and often hidden by a calcareous bed of a vitreous appearance.
It is now known that the form and colouring of the shells vary very considerably, according to the age of the animal: so much so, indeed, that the same species examined at various stages of its growth would almost seem to belong to species and even to genera essentially different.
The young cowries are thin, conical, elongated; with conspicuous spiral, and large openings. The right edge soon becomes thicker, and folds itself inwardly; the opening is narrowed; finally, the spiral is unfolded in successive folds from the right edge, and by successive deposits of the vitreous matter we have spoken of the opening is gradually contracted, its extremities hollowed out, its edges disconnected, and the shell, until now only shaded in pale tints, assumes its most brilliant colours, disposed in bands or spots, as exhibited in Pl. XXII., in which Figs. I. and II. are the adult shells, and Fig. III. the young shell, of Cypræa Scottii.