THE MOLLUSCA 



993 



95 (97) Shell elongated, conic, or cerithiform; aperture subrhomboidal, pro- 

 longed into a short canal below; columella twisted, not 

 callously thickened. . . . Pleurocera Rafinesque . . 96 



Exceedingly abundant and of great variety. Numerous species have 

 been described from the rivers of Kentucky, Tennessee, and Alabama. A 

 few species extend north to the Great Lakes and west to the Mississippi 

 Valley. The species vary greatly in contour, ranging from long, slender, 

 and rather thin to large, heavy, and broadly conic. Example, P. canalicu- 

 latum Say (Fig. 1473). 



Fig. 1473. 



96 Shell smooth; spire obtusely conical; body- whorl subcyUndrical ; aper- 



ture subquadrate; columella thickened below, twisted and 

 drawn back, base subcanahculate; lip very sinuous. 

 Vv Section Strephobasis Lea. 



A niunber of nominal species have been described from Tennessee and 

 northern Alabama. Type, Pleurocera plena Anth. (Fig. 1474). 



Shell ovate-conic to elongate; smooth, plicate, striate, or tuberculate; 

 aperture subrhomboidal, subangular at the base, but not 

 canaliculate; columella simple, smooth. . . Gofiiobasis \^t^. 



This genus comprises about three-fifths of all the species of the family 

 and is enormously developed in the rivers of Tennessee and Alabama. A 

 few species extend north to the St. Lawrence Valley and west to Texas and 

 the western tributaries of the Mississippi. A small group of species is also 

 found on the Pacific coast and is the only genus of the family represented in 

 that region. Example, G. virginica Gmel. (Fig. 1475), from the Atlantic 

 states. 



Fig. 1474. 

 97 (98) 



Fig. 1475- 



98 (99) Shell conical or globose-ovate; aperture with a slit along the suture, 

 entire below Gyrotoma Shuttleworth. 



This remarkable genus is confined to the Coosa River, Alabama, where 

 it is represented by a considerable number of described species. The 

 sutural sHt is characteristic and is either direct, narrow, and deep, or ob- 

 lique, short, and wide. Example, G. demissum Lea (Fig. 1476). 



Fig. 1476 



