This genus occupies an intermediate space between the Trochus and the Turbo; distinguished from the former by the aperture being more round and slightly depressed, and from the latter by the toothlike projecting angle which the truncated columella occasions at the base of the aperture. Shell ovate or conoid, aperture round and entire, with an operculum; outer lip disunited from the body at the top; columella arched and truncated at the base. Monodonta bicolor. M. pagodus. M. tectum Persicum. M. papillosa. M. coronaria. M. Egyptiaca. M. carchedonius. M. tectum. M. labio. M. Australis. M. canalifera. M. viridis. M. fragarioides. M. constricta. M. modulus. M. articulata. M. lugubris. M. punctulata. M. tricarinata. M. canaliculata. M. Seminigra. M. rosea. M. lineata. _M. coronaria._ The crowned Monodonta. Species in which the columella greatly projects, and the spire is entirely flat; covered with numerous small tubercles; colour white; the columella tinged with red. _M. labio._ The double-lipped Monodonta. Pl. 23, fig. 2. Species sub-globular, umbilicated, spiral whorls rounded; the columella terminated by a tooth. _M. fragarioides._ The strawberry-shaped Monodonta. Species more or less globular, of which the columella, almost straight, offers but a little obstacle to its junction with the margin. 4. Turbo. The Turban Shell. Thirty-four species. Distinguished from the Monodonta by never having the columella truncated at the base; and from the Trochus by being solid, with the whorls constantly convex and never flattened. Like the Trochus, when decorticated, the Turbo exhibits splendid pearly, gold, or silver irridescent colours. Shell thick, pearly in the interior; depressed, conical, or sub-turreted; umbilicated or not, little or not carinated at its circumference; aperture round or little depressed; the middle of the external edge not bent, but sometimes hollowed or sloped in some part; the edges rarely joined by a callosity; the columella arched, rarely twisted, and not truncated at the base; operculum calcareous or horny; the spire visible externally in the latter and interiorly in the former; the exterior often thickened and curved.