376 SCUTIBRAKCIIIATA. 



Genus NERITOPSIS, Grateloupe. 



Shell spiral, ob ovate, white, solid; spire short, whorls 

 rounded, cancellated ; aperture semilunar ; inner lip thick, 

 rather flat, with a large, square notch in the middle ; outer 

 lip internally thickened and grooved. 



Syn. Radula, Gray, not Klein. 



Ex. N. radula, Linnaeus, pi. 41, fig. 6. 



The animal of this genus is not known, but from the 

 peculiar white appearance and sculpture of the shell it 

 appears to belong to this family ; from the operculum 

 never having been met with, it is probably very thin, and 

 has escaped the notice of collectors. The recent species 

 is from the Pacific, and there are about twenty fossil 

 species. 



Order SCUTIBRANCHIATA. 



Gill consisting of two series of lamell8e, forming one or 

 two series over the back of the neck, or on the under edge 

 of the mantle, round the foot. Animal hermaphrodite. 

 Shell spiral, or symmetrical and conical. 



The Scutibranchiate Gasteropods comprise a vast as- 

 semblage of Mollusks, all marine, and chiefly littoral in 

 their habits, living upon the sea-weed covering the rocks 

 along the shore. They are hermaphrodite and self-im- 

 pregnating, the sexes not being separate in different in- 

 dividuals, as in the Pectinibranchs. In the Neritidce, 

 TrochidcB, and Fissurellidce the heart is traversed by the 

 rectum, while in the Tecturidae, Gadiniidm, Patellidm, 

 and ChUonidce, the heart is separate from the rectum. 



