CONCHOLOGY. 155 
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. Zin.) 
2. Genus Wautilus. Pl. XII. 
Animal. Body round and terminated posteriorly by a tendi- 
nous 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. In- 
habits the Indian Ocean. Two living species. Fifteen fossil. 
Nautilus Pompilius. Nautilus umbilicatus. 
FAMILY XXIII. 
Hetreroropa. 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, bicari- 
nated, longitudinally subinvolute in the same plane; aperture 
very wide, symmetrical, complete, square anteriorly, slightly mo- 
