986 FRESH-WATER BIOLOGY 



59 (60) Shell large, thick and solid; apex smooth Lanx Clessin. 



This genus is restricted to the Pacific coast and is distinguished by 

 the large size and thick soUd shells. Type, L. newberryi Lea.. (Fig. 

 1443). 



Fig. 1443. 



60 (61) Shell ancyliform, small, thin, with a septum across the apical portion 



of the interior Gundlachia Pfeifier. 



This very remarkable and pecuUar genus has a general but very local 

 distribution from the Atlantic to the Pacific. Example, G. meekiana 

 Stimp., (Fig. 1444; X 6), from the Eastern States. 



Fig. 1444. 



61 (62) Shell small, spiral, dextral, neritoid, or crepidtila-Uke, with a broad, 

 thin, columeUar plate projecting across the end of the aper- 

 ture next to the spire Amphigyra Pilsbry. 



Only a single species, .4. alabamensis Pils., (Fig. 1445; X 10), from the 

 Coosa River, Alabama, is known. 



Fid. 1445- 



62 (54) Shell very minute, dextral, spiral, subdiscoidal; columeUar margin 

 broadly dilated Neoplanorbis Pilsbry. 



Four species of this genus have been recently described from the Coosa 

 River, Alabama, and are among the smallest moUusks known in our 

 fauna. Type, N. tanlillus Pils., (Fig. 1446; X 10). 



Fig. 1446. 



63 (2, 100) Animal operculate, branchiferous (except Assimenia). Radula 



with seven rows of teeth. 



Order Pectinibranchiata. Suborder Taenioglossa . . 64 

 Six families: 64, 65, 66, 71, 72, 90. 



64 (65) Shell small, spiral, dextral, conical; operculum spiral. Animal pul- 

 moniferous Family Assimeniidae. 



Only a single genus Assimenia Leach. 



I It Ull'* "^^^ Assimenias live in brackish water in the upper part of the littoral zone. 



Two species occur on the Florida keys and two on the coast of California. Ex- 

 ample, A. caUfornica Tryon, (Fig. 1447; X 4). 



Fig. i447> 



