74 CONCHOLOGIA CESTRICA. 



A. t^rdus, SAY, New Harm. Diss., V., 1830. 

 A..tardus. Shell, conic, white, opaque; apex almost 



r\v c* 1^1 



Fig, 151, terminal, inclining backwards ; anterior slope 

 rounded ; posterior straight, almost perpen- 

 dicular; sides equal, steep, curved; aperture 

 broad, oval, not narrowed at one end. H. 3, 

 L. 5, W. 3 J4, 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 VIVIPARID^. 



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. 



