Bryant Walker 

 Genus PISIDIUM C. Pfeiffer, 1821. 



Shell small, rounded, oval or obliquely cuneiform; inequilateral, anterior 

 side longer; beaks terminal; cardinal teeth double in each 

 valve, at times united, situated immediately under the beaks; 

 laterals elongated, lamelliform, double in the right, single in 

 the left valve ; ligament on the shorter side, internal. 



Animal with a single siphon, the excurrent orifice being 

 merged with that of the foot. 



Fig. 231 



Type : Tellina amnica Miill. 



Example: P . virginiciim (Gmel.), fig. 231. 



Family CYRENELLID^. 



Shell rounded, inflated, thin, with a conspicuous epidermis; beaks for- 

 ward ; cardinal teeth two on the right and one on the left valve ; no lateral 

 teeth; pallial line not sinuate. Animal with two contractile siphons, elon- 

 gated and united to their tips. 



Genus CYRENELLA Deshayes, 1835. 



Shell as in the family, surface smooth or slightly con- 

 centrically sculptured. 



Type : Cyrenoidea dupontice Joannis. 



Example: C. Horidana (Dall), fig. 232. 



Fig. 232 



Family RANGIID^. 



Shell equivalve, covered with an epidermis, no internal nacre; beaks 

 prominent, separated; hinge with two cardinal and anterior and posterior 

 lateral teeth in each valve and an internal, central cavity for the ligament; 

 pallial line sinuous. Mantle with two short siphons united at their bases 

 and with papillose orifices ; foot large, linguiform, compressed. 



Fig. 233 



Genus RANGIA Desmoulins, 1832. 



Shell thick, oval, subtrigonal, ventricose, 

 smooth, epidermis olive, beaks prominent, sep- 

 arated ; ligament and resilium both enclosed in 

 a single pit and invisible externally; laterals 

 curved, cross-striated, more or less unequal, 

 the posterior longer, anterior with the prox- 

 imal end vertically hooked ; pallial sinus small. 



Type : Gnathodon cnneatns Gray, fig. 233. 



