_Animal._ Body elongated, cylindrical, terminated posteriorly by two lateral lobes partially concealing the shell; head provided with five pairs of tentacula, of which two are longer than the others. _Shell._ Very symmetrical, longitudinally twisted throughout nearly all its extent; the cone spiral, conical, regular, circular; whorls of the spire very evident; partitions simple, concave, and pierced by a single syphon. Inhabits the West Indian seas. One species. Spirula Peronii. (Nautilus spirula. _Lin._) 2. Genus _Nautilus_. Pl. XII. _Animal._ Body round and terminated posteriorly by a tendinous or muscular filament, attaching itself to the syphon, by which the partitions of the shell are pierced; the mantle opening obliquely, and prolonged into a sort of hood above the head (which is provided with digitated tentacula), and surrounding the aperture of the mouth. _Shell._ Discoid, but slightly compressed, with a rounded or subcarinated back, umbilicated or not, but never mammelonated, the partitions simple, not visible exteriorly; the last profoundly sunk, and perforated by a syphon running through them all. Inhabits the Indian Ocean. Two living species. Fifteen fossil. Nautilus Pompilius. N. umbilicatus. FAMILY XXIII. HETEROPODA. Two genera. 1. Genus _Argonauta_. Pl. XII. _Animal._ Body conical, elongated, enrolled longitudinally, widened anteriorly, and provided on each side with an arcuated subtriangular, aliform appendage; mouth at the extremity of the angle formed by two inferior lips. De Blainville denies that this animal is at all known, and speaks of one described by M. Oken as a small polypus of the genus Ocythoe. _Shell._ Navicular, symmetrical, very thin, compressed, bicarinated, longitudinally subinvolute in the same plane; aperture very wide, symmetrical, complete, square anteriorly, slightly modified by the turn of the summit, and provided on each side with an ear-like appendage having thick and smooth edges. Inhabits the Mediterranean. Three species. Argonauta argo. A. tuberculosa. A. nitida. 2. Genus _Carinaria_. Pl. XII. _Animal._ Body elongated, prolonged behind the nucleus into a veritable tail, edged at its extremity by a vertical fin; head sufficiently distinct; two, long, conical tentacula; two sessile eyes; the organs of respiration and the nucleus entirely enveloped in a mantle with lobed edges. _Shell._ Very thin, symmetrical, a little compressed, without spire, but with the summit a little reflexed posteriorly; aperture oval and entire. Inhabits the African, Mediterranean, and Australian seas. Three species. Carinaria vitrea. C. fragilis. C. cymbium. GLOSSARY OF TERMS USED IN CONCHOLOGY. A. Abbreviated, cut short. Abdomen, the belly. Acuminated, sharp pointed.