74 CONCHOLOGIA CESTRICA. 
A. tardus, Say, New Harm. Diss., V., 1830. 
A, tardus. Shell, conic, white, opaque; apex almost 
(W-G Pl terminal, inclining backwards; anterior slope 
Fig, 161, g Ie 
rounded; posterior straight, almost perpen- 
dicular; sides equal, steep, curved; aperture 
broad, oval, not narrowed at one end. H. 3, 
L. 5, W. 3%, mill. 
Station, in stagnant pools. New Garden, Chester 
County ; rare. 

OPERCULATA. 
With an opercle. 
Order PECTINIBRANCHIATA. 
Animal, with the gills arranged in numerous, parallel, 
lamina. | 
Sub-order ROSTRIFERA. 
Animal, fluviatile or marine ; head produced; rostrum 
contractile ; tentacles subulate ; operculate. 
Family VIVIPARIDA. 
Shell, conoid; whorls convex ; aperture ovate or sub- 
rotund; epidermis green, or olivaceous. 
Animal, viviparous ; rostrum small, simple, truncated, 
extending slightly beyond the shell; tentacles short, 
subulate, the right one, on the male, as large as the 
rostrum; eyes on peduncles at the exterior base of the 
tentacles; foot large; operculum corneous, concentric, | 
sometimes with a spiral nucleus. 
