Shell oval or sub-globular, thick, armed with points or tubercles, with a depressed spire; aperture narrow, elongated, notched, sometimes canaliculated anteriorly, and digitated externally; the left edge more or less callous, sometimes denticulated; operculum horny, oval, transverse, with elements slightly imbricated. Ricinula horrida. R. clathrata. R. arachnoidea. R. miticula. R. digitata. R. pisolina. R. aspera. R. morus. R. mutica. _R. horrida._ The horrid Ricinula. Pl. 26, fig. 2. Species without a canal; exterior covered with strong, obtuse black tubercles, with the interstices white, striated transversely; interior rich purple colour; outer lip with five triangular grooved radii, between which at their base the margin is crenulated. _R. digitata._ The digitated Ricinula. Species canaliculated; two long palmated digits at the side of the aperture. 4. Purpura. Fifty species. This genus has its name from the purple liquid produced by the animal, from which the ancients extracted the Tyrian purple dye. This is the last genus that presents the appearance of a canal at the base of the aperture, and therefore rightly precedes the remaining genera of this family, all of which are without a canal. Shell oval, thick, generally tuberculated; spire short; the last whorl much greater than all the others united; aperture oval, greatly dilated, terminated anteriorly by a canal short, oblique, and notched at the extremity; the columellar edge almost straight, covered with a callosity pointed anteriorly; operculum horny, flat, almost semicircular, with transverse striæ slightly marked; the summit behind.