93 



more important and better characters for the separation of the 

 families, genera and sp icies, than the operculum. This lias been 

 illustrated in the family Neritida. 



The family is well characterized by tin- form of this part, and the 

 possession of the internal apophysis or Bhelly lobe under the nucleus 

 forming a kind of binge on the sharp inner lip of the shell. 



In my paper in the 'Philosophical Transactions' for 1833, 



I stated that the structure of the operculum offered the best 

 character to separate the Neritts from the Neritina, and I there ob- 

 served, "The operculum of Nerita agrees in form with that of A - 

 ritina, but differs in haying do cartilage on its edge, which is fur- 

 nished instead with a groove in its outer surface, being com red with 

 a thick, variously formed Bhelly deposit as in the genus Turbo, and 

 in its inner surface being lined with a thick, callous, polished coat. 

 Between the outer and inner coat there exists B very distinct concen- 

 trically striated horny layer, like the operculum of Littorina, and 

 the left muscular scar is deeply grooved like that of the suhannular 

 operculum. 



" This difference in the structure of their opercula forms an ex- 

 cellent distinctive character between these two genera." 



In the same paper I observed, " The difference in the outer sur- 

 face of the opercula of the genus Nerita affords a good character 

 for the separation of the species." 



I have lately had an opportunity of examining a large number 

 of freshly collected Nerites, with their opercula dried in the mouth 

 of the shell, so that there can he no doubt that they are the real 

 opercula of the species, and that these opercula have not been put 

 into the mouths of the shells at random, as is too often the case 

 with shells which have passed through the hands of dealers *. 



The species may be divided according to their opercula as fol- 

 lows : — 



1. Operculum polished, with abroad, slightly raised, concentrically 



grooved, submarr/inal band. Nerita. 



N. polita. Costal grooves arched (fig. 1). 

 N. lineolata. Costal grooves straight (fig. 2). 



2. Operculum polished, with a brum/, slightly raised, uninitiated, 



submarginal buml. Etitena. 



N. plicata (fig. 3). 



The specimens var] slightly in the distinctness, and especially in 



the hreadth, of the tubercular Bubmarginal band. 



3. Operculum with a broad, raised, < narguuU 



bam/. Tenare. 



* Operculum smooth. 



N. Peloronia (fig. .'<). 



* In Adams's Onera of Shells, t. 12. f. 1, n,b, a granular operculum, probably 

 that of .V. tignata, i| IgQTOd i> th.it of Xeritn / 



