994 



FRESH-WATER BIOLOGY 



99 (91) Shell thick, solid, subglobose, with a very short spire, or thinner and 



conical; aperture oval or subcircular, entire below; columella 

 callously thickened -. : * . Anculosa Say. 



This group differs from all of the genera of the family by the entire aperture. 

 The heavy, subglobose species range from the Ohio River south into Alabama and 

 Georgia but are not found in the northern Atlantic States nor in the Mississippi 

 Valley. The thin, conical species are characteristic of the Atlantic drainage 

 from New York southward. Type, A. praerosa Say (Fig. 1477). 



FIG. 1477. 



100 (2, 63) Radula with numerous rows of teeth, consisting of a central 



tooth, 2-5 laterals, and numerous marginals arranged like the 

 sticks of a fan. 



Order Aspidobranchia . . Suborder Rhipidoglossa. 

 Represented by a single family. . . NERITIDAE . . 101 



101 (102) 



FIG. 1478. 



Shell globose, imperf orate, very thick and solid; aperture semi- 

 ovate, columellar region expanded, flattened, and thick- 

 ened; operculum calcareous, edge with projecting processes 

 (apophyses), articulating with the columella. 



Neritina Lamarck. 



A few species of this characteristic tropical genus are found in the fresh and 

 brackish waters of Florida and the Gulf coast. Example, N. reclivata Say (Fig. 

 1478). 



102 (101) Shell small, thin, corneus; columella concavely flattened, calloused; 

 operculum corneus, paucispiral, without apophyses. 



Lepyrium Dall. 



This genus was created for a single small species, known only from the 

 Coosa and Cahawba rivers in Alabama and is peculiar in the character of 

 the operculum. Type, L. showalteri Lea (Fig. 1479; 



FIG. 1479. 



103 (i) Animal acephalous. Shell consisting of two opposing, symmetrical 

 valves united by a ligament. Class Lamellibranchia . . 104 



Represented by a single order, Eulamellibranchia . .104 

 Seven families: 105, 106, 166, 167, 173, 174; in two groups: 104, 165. 



104 (165) Shell equivalve; interior nacreous; ligament external; hinge with 



or without teeth, but never with true cardinal teeth; when 



present, the modified anterior lateral teeth are known as 



pseudocardinals and the posterior teeth as laterals. . . 105 



Two families: 105, 106. 



