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.