_Solarium_ _Trochus_. _Rotella_ _Trochus_--wheel-shell. _Trochus_ _Trochus_--top shell. _Monodonta._ _Turbo_ _Turbo_. _Planaxis_ _Buccinum_. _Phasianella_ _Turbo_. _Turritella_ _Turbo_. "The genus _Rotella_ contains small, flattened, wheel-shaped shells, common in most collections. They are smooth and polished. "_Trochus_ is still a large genus. _Trochus marmoràtus_ is a fine species from the Indian Ocean. There are several handsome species on our own shores. _T. magus_ is one of them; it has a large, deep umbilicus, or perforation; the spire is flattened; the whorls are crowned with tubercles. The _Trochi_ of tropical climates are thinner than those of northern latitudes. "When any of these shells are placed upon their _base_, their axis is always inclined: of course they never stand perfectly upright. "There are several fossil species. "The genus _Monodonta_ holds a middle place between _Trochus_ and _Turbo_, differing from the former in the aperture, and from the latter in the columella, which is arched and truncated at the base. They are all marine shells. [Sidenote: LITORINA. TURBO. PHASIANELLA.] "From the well-known genus _Turbo_, a new one has been formed, called _Litorina_, which includes all the shells of our own coasts that formerly ranked under _Turbo_. Consequently we find the periwincle has changed its generic name, and from _Turbo_ it is altered into _Litorina_. _T. muricata_ is now of the same new genus, _Litorina muricata_. (Plate 7.) "_Turbo pica_ is a large pearly shell known as the magpie. The golden-mouthed _Turbo_ is very brilliant; the aperture appears as if gilded, so fine is the yellow tint. It comes from the Molucca Isles. _Turbo smaragdus_, from New Zealand, is a rare and beautiful species of a bright green colour. "_Phasianella_ is a beautiful genus of shells, formerly very costly. A small but elegant species is found on our own shores, _P. pullus_, _Turbo pullus_ of some authors. The colour is pink. "_Phasianella bulimoides_, from New Holland, is the largest of the species, and once a very rare shell. "The term _Turritella_ will give you an idea of the form of our last genus in the family _Turbinacea_. The shells are like _little towers_, with a circular aperture. The older conchologists gave the name of _screw_ to all turreted shells, without attending to the form of the aperture. Hence we find screws among _Turbo_, _Buccinum_, and _Strombus_ (spindle). [Sidenote: TURRITELLA.] "_Turritella duplicata_ is a heavy shell, often more than four inches long. It is sulcated and carinated; the colour is yellow-white. _T. bicingulata_ is white, marbled with yellow; the whorls are girded with two ridges. There are several fossil species of this genus in London clay."