MARINE MOLLUSCA OF THE UNITED STATES. 9 
trace of eyes. Gills plumose, partially exserted. Foot small, 
flat, rudimentary, furnished with a vesicular appendage on the 
hinder part. Pelagic. 
The shell is very thin, translucent, with a sinistral nucleus. 
Violet colored. The family contains but one genus. 
Genus JANTHINA, Bolten. 
Mus. Bolt. 1798. 
1. J. FRAGILIS, Bruguiere. Fig. 182. 
Helix Janthina, Gmelin, Syst. Nat. 3645. 1790. 
Janthina communis, Lam. Anim. s. Vert., vii. 206. 1822. 
Shell depressly semiglobose, flattish beneath; whorls slopingly 
convex, rather rudely decussately striated; whitish above, violet 
below, whitish around the columella; aperture transverse; a little 
sinuated in the middle. 
Driven upon the Northern Shores during storms. 
Family FISSURELLID. 
Animal with a well-developed head, a short muzzle, subulate 
tentacles, and eyes on rudimentary pedicels at their outer bases ; 
sides ornamented with short cirri; branchial plumes two, symme- 
trical; anal siphon occupying the anterior notch or perforated sum- 
mit of the shell. 
Synopsis of Genera. 
Shell oval, conical, depressed, with the apex in front of the centre, and per- 
forated ; surface radiated or cancellated ; muscular impression with the 
points incurvyed. FIssURELLA, Bruguiere. 
Shell conical, elevated, with the apex recurved ; perforation in front of the 
apex, with a raised border internally ; surface cancellated. 
Cremorta, Leach. 
Genus FISSURELLA, Bruguiere. 
Encyc. Meth. i., 14. 1789. 
1. F. ALTERNATA, Say. Fig. 183. 
Journ. Philada. Acad. Nat Sci., ii. 224. 1822. 
Shell oblong-ovate, moderately thick, cinereous or dusky, with 
equal concentric lines crossed by alternately larger and smaller 
radii, all of which are équable or not dilated in any part; vertex 
placed nearer the smaller end; perforation oblique, oblong, and a 
little contracted in the middle; within white; margin simply cre- 
