CONCHOLOGY. 125 



N. mamilla. The nipple Natica. 



Species with the umbilicus entirely covered over by a large 

 callosity ; the spire papillose, and the operculum horny. 



5. Janthina. Two species. 



Formerly classed with the Helix, which it somewhat re- 

 sembles in form, but properly separated, as it differs in every 

 other respect. 



It is a singular marine shell, often found in great numbers 

 floating on the surface of the sea, suspended by a vesicular 

 appendage, which stains the hand of a purple colour. 



Shell sub-globular, ventricose, extremely thin and fragile ; 

 transparent, of a beautiful violet colour ; the spire low, lateral, 

 pointed, with sub-carinated whorls ; aperture large, sub-an- 

 gular, greatly modified by the last whorl of the spire ; edges 

 disunited, the left entirely formed by the columella, which is 

 straight and continued beyond the base, the right edge sharp, 

 often with a sinus in the middle. 



Janthina communis. Janthina exigua. 



/. communis. The common Janthina. PI. 18, fig. 6. 

 Very fragile, aperture triangular, with a small notch on 

 the margin of the outer lip ; beautiful violet colour. 



FAMILY XII. 



Macrostomides. Four genera. 



1. Sigaretus. Four species. 



Distinguished from the Natica by the great width of the 

 aperture, and its short spiral columella. 



Shell oval, more or less thick, very depressed, spiral short, 

 little elevated, lateral; aperture very extended, entire, the 

 left edge reflected and sharp ; two lateral muscular impres- 

 sions very disunited. 



Sigaretus Haliotoideus. Sigaretus laevigatus. 



S. convexus. S. cancellatus. 



