Dr. J. E. Gray on the genus Nerita. 65 



vary considerably in number and size, and are sometimes almost 

 entirely wanting. It is to be observed that in many of the species 

 which have this part tubercular, the tubercles are niore distinct and 

 crowded in the younger, and especially the youngest, than in the 

 older, or what is usually called the more perfectly developed state of 

 the species. In other genera such characters are generally more 

 developed in the shells formed in the most perfect state of the 

 animal. :Mr, Adams has founded subgenera on characters furnished 

 by the surface of the inner lip. 



My studies on Mollusca have proved to me that few parts offer 

 more important and better characters for the separation of the 

 families, genera and species, than the operculum. This has been 

 illustrated in the family NeritUlcB. 



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

 possession of the internal aj)ophysis or shelly lobe under the nucleus, 

 forming a kind of hinge on the'sharp inner Up 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 Nerifce from the Neritinai, and I there ob- 

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

 ritma, but differs in having no cartilage on its edge, which is fur- 

 nished instead with a groove in its outer surface, being covered with 

 a thick, variously formed shelly deposit as in the genus Turijo, and 

 m its inner surface being hned with a thick, callous, polished coat. 

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

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

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

 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 be 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 a broad, slightUj raised, concentrically 

 grooved, submarginal band. Nerita. 



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

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



* In Adams's Genera of Shells, t. 42. f. 1, a, 6, a granular operculum, probably 

 that of A^. signata, is figured as that of Nerita polita. 



Ann. i^ Mug. N. Hist. Scr. 3. Vol. ii. 5 



