We have a few common species belonging to this group, mostly members of the typical genus Trochus and commonly known as Top Shells. In these the shell is a pyramid formed of numerous flat whorls, with an oblique and rhomboidal aperture. Of the three species figured (including two on Plate V.) T. umbilicatus and the Large Top (T. magnus) are umbilicated, the umbilicus being very large in the latter; and the former is characterised by the zigzag greyish or reddish markings that run radially across the whorls. The other (T. zizyphinus) is usually of a yellowish or pink colour and has no umbilicus.
The same family contains the pretty little Pheasant Shell (Phasianella pullas), which is richly coloured with red, brown, and yellow on a light ground; and Adeorbis subcarinatus, shown in the same group.
Fig. 171.—1. Trochus zizyphinus. 2. Under side of Shell. 3. Trochus magnus. 4. Adeorbis subcarinatus
The well-known Periwinkle (Littorina littorea) and the species to the right of it on [Plate V.], belong to the family Littorinidæ, the members of which are similar in structure and habit to Trochus, but the shell is usually more depressed, and is never pearly. The shell of the Periwinkle is thick, having but few whorls, and is not umbilicated; and the lingual ribbon, which is coiled up on the gullet, contains no less than about five hundred rows of teeth; but only a little more than twenty of these rows are in action at any one time, the remainder being a reserve stock to come into active service as the ribbon grows forward. In the genus Lacuna there is a narrow umbilicus, and the aperture of the shell is semilunar in form; and the species of Rissoa are very small, with white or horny shells, much more pointed and having more whorls than those of the Littorina.
Fig. 172.—Rissoa labiosa and Lacuna pallidula
Our next illustration shows three shells of the family Turritellidæ, so named from the resemblance of the shells to a tower or spire. The form indeed is so characteristic that they can hardly be mistaken. It will be seen that Turritella communis is striated spirally, while the surface of Scalaria communis (Plate V.) is relieved by strongly marked transverse ribs. Both these species are very common, and the latter is peculiar for its power of ejecting a dark purple fluid when molested. The other representative of the family—Cæcum trachea—has a shell something like that of Dentalium (p. 238), being cylindrical and tubular, but it differs in being closed at one end.