CONCHOLOGY. 145 



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 

 spedes. 



Spirula Peronii. 



2. Genus JVaiitihis. PI. 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 

 subcarenated back, umbilicated or not, but never manielonated, 

 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 Umbilicatas. 



FAMILY XXIII. 



Heteropoda. Two genera. 



1. Genus ^rgonauta. PI. 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- 

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