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. The well-known Periwinkle (_Littorina littorea_) and the species to the right of it on Plate V., belong to the family _Littorinidœæ_, 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_. <div class="figleft" style="width: 175px;"> <a name="fig172" id="fig172"></a> <img src="images/i_273b.jpg" width="175" height="202" alt="" /> FIG. 172.--_Rissoa labiosa_ AND _Lacuna pallidula_ </div> Our next illustration shows three shells of the family _Turritellidœæ_, 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. [Illustration: FIG. 173.--SECTION OF SHELL OF _Turritella_] [Illustration: FIG. 174.--_Turritella communis_ AND _Cœæcum trachea_]