THE MOLLUSCA 



1019 



170 (171) Shell thin and delicate, suborbicular to oblong; beaks prominent, 

 usually retaining the nepeonic valves; cardinal teeth minute, 

 often obsolete; anterior and posterior laterals present. 



MiiscuUum Link. 



This group has a general distribution. The prominent beaks with 

 the distinctly mari^ed nepeonic shell are the distinctive feature, but in 

 some species these are lacking. The thin, rounded, polished shell is, 

 however, quite characteristic. E.xample, M. partumeium Say (Fig. 



Fig. 1530. 



171 (172) Shell subrhomboidal, thin, moderately inflated, with the posterior 

 side longer; cardinal teeth feeble, only one in each valve. 



Eupera Bourguignat. 



A tropical group, of which two or three species occur in Florida, Ala- 

 bama, and Te.xas. The rhomboidal shape is characteristic. The shells 

 appear to be mottled and are usually so described, but according to 

 Dr. W. H. Dall these "spots" are caused by a parasitic infusorian that 

 attacks the interior of the shell. Example, E. singlcyi Pils. (Fig. 1531; 



Fig. isji. 



172 (169) Shell small, rounded, oval, or obliquely cuneiform, inequilateral, 

 anterior side longer; beaks sub terminal; carchnal teeth 

 double in each valve. Pisidium C. Pfeiffer. 



The Fisidia are of general distribution and a great number of species 

 have been described. They arc easily distinguished from the allied 

 genera by the very inequilateral shell, the hinge being on the shorter 

 side. E.xample, P. virginkum Bgt. (Fig. 1532; X 2). 



Fig. IS3S- 



173 (174) Shell rounded, inflated, thin; beaks forward; surface smooth or 

 slightly concentrically sculptured; cardinal teeth, two in 

 the right and one in the left valve; no lateral teeth. 



Family Cyrenellidae. 

 Only one genus Cyrenella Deshayes. 



Fig. 1533- 



Represented in our fauna by a single species, C. floridana Dall (Fig. 

 IS. 1.5; X ij), from Florida. It is easily distinguished by the lack of lateral 

 teeth. 



