FAMILY XVII. ALATA. Three genera. 1. Rostellaria. Three species. Lamarck formed this genus from the Strombus of Linnæus on account of having a sinus in the lower part of the right margin contiguous to the canal. The beak is generally curved, and short in comparison to the length of the spire, but sometimes it is straight, and equal in length to the other part of the shell. The R. rectirostris is one of the most rare shells known. Shell sub-depressed, turreted, with spire produced and pointed; aperture oval by the excavation of the columellar edge; the right margin dilating by age, and having a sinus contiguous to the pointed canal which terminates the shell. Rostellaria curvirostris. R. rectirostris. R. pes-Pelicani. _R. curvirostris._ The curved beak Rostellaria. Pl. 27, fig. 2. Species with the right edge digitated. _R. pes-Pelicani._ The Pelican’s foot Rostellaria. Species turreted, with four digitations on the right edge; body and volutions ribbed longitudinally and crowned with papillæ; flesh-coloured or white. 2. Pteroceras. Seven species. Formed from the Strombus, being distinct from it by not having the canal at the base shortened or truncated. It greatly resembles the Rostellaria, but the sinus of the right margin is distant from the body. From its digitation or long recurved claws it has often been called the Spider Shell. Shell oblong-ovate, ventricose, canal elongated, attenuated, and often closed; right margin dilating by age into an expanded, digitated wing, attached to and covering a short spire, with a sinus in the lower part not contiguous to the body. Pteroceras truncata. P. lambis. P. millepeda. P. pseudoscorpio. P. scorpio. P. aurantia. P. chiragra. _P. chiragra._ The Devil’s Claw. Pl. 28, fig. 3. Tuberculated, with six digitated, canaliculated rays, closed in the adult shell; outer lip internally striated. _P. scorpio._ The Scorpion Pteroceras. Species with digitations on the external edge, varying in number from six to ten. 3. Strombus. The Wing Shell. Thirty-two species. As now defined and characterized by Lamarck, is easily distinguished by not having the winged aperture on the right side dentated or digitated, and the sinus therein always separated from the canal. In some species the exterior is variously striated, smooth, wrinkled longitudinally, or tuberculated; the interior presents vivid and beautiful colours. These shells frequently attain a large size and great solidity.