of mollusca. 85 



1. Stoastoma. 



Operculum annular, calcareous, externally very deeply concave, 

 with lamellae slightly and irregularly projecting from the surface. 

 Shell globose-conic, depressed or discoidal ; aperture semioval or 

 nearly semicircular, angulated above and beneath. Peristome con- 

 tinuous, with the right margin arcuate, left margin nearly straight, 

 sending off at its base a more or less distinct keel, which surrounds 

 the more or less open perforation. 



* Shell smooth. Electrina. 



1. S. succineum. 



** Shell spirally costate. 



2. S. pisum. 



2. Trochatella. 



Operculum annular, semioval, flat, pale, more or less solid. Shell 

 top-shaped, or globose-conic ; aperture semioval, rather triangular ; 

 columella nearly flat, not sending backward any basal callus, some- 

 times uniting the margins of the peristome by a very thin deposit, 

 which is not diffused. 



1. T. pulchella. 



3. LuCIDELLA. 



Operculum membranaceous, semiovate, with the columellar margin 

 scarcely thickened. Shell depressed, heliciform, not callous beneath ; 

 aperture sinuate, rather triangular. Peristome thick, furnished with 

 stout teeth. 



1. L. aureola. 



4. Helicina. 



Operculum semiovate, membranaceous or shelly. Shell heliciform, 

 turbinate, globose or depressed, covered beneath with callus round 

 the columella, which is rather straight and somewhat flattened ; 

 aperture triangular or semiovate, entire. Peristome simple, straight 

 or thickened, often spreading broadly. 



a. Shell smooth. Peristome more or less toothed internally. 



1. H. depressa. 



b. Shell smooth. Peristome not toothed. 



2. H. neritella. 



3. H. flammea, t. 311. f. 3. 



4. H. tseniata, t. 311. f. 1. 



5. H. Sandwichensis, t. 303. f. 10. 

 G. H. oresignse, t. 311. f. 6. 



7. H. zephirina, t. 311. f. 2. 



8. H. variabilis, t. 290. f. 9, t. 311. f. 7. 



9. H. occidentalis, t. 311. f. 4. 



Tongue -membrane of Helicina agglutinans rather broad, thin. 



