THE MOLLUSCA 



995 



105 (106) Shell elongated, laterally compressed; hinge with usually only 

 pscudocardinals; laterals, when present, very obscure. Gills 

 without water tubes and with scattered interlamellar con- 

 nections, which in certain places form irregular diagonal rows. 



Family jMargaritanidae. 

 Only one genus Margaritana Schumacher. 



The typical species, M .margaritiJera'L. (Fig. 1480; X?), is circumboreal, but in this country is 

 found only in the northern Atlantic and Pacific states, being unknown, with one possible excep- 

 tion, from the whole interior portion of the continent. Another species is found in the Ten- 

 nessee and Ohio drainage systems, and two more have been described from the Gulf drainage. 



Fig. 1480. 



106 (105) Shell subcircular, oval, subtriangular, or elongated ; hinge edentulous 

 or with pscudocardinals only or with both pscudocardinals 

 and laterals. Gills with water tubes and distinct, contin- 

 uous interlamellar septa, running parallel to the filaments. 



Family Unionidae . . 107 



107 (121, 140) Marsupium formed by all four gills or by the outer gills only; 

 edge of marsupium always sharp and not distending; water 

 tubes simple in the gravid female. 



Subfamily Unioninae . . 108 



Five genera: 108, 



13, 114. 115, 117 



108 (113) All four gills serving as marsupia. Shell alike in both sexes, tri- 

 angular, quadrate or rhomboidal, solid, inflated, beaks 

 usually prominent, sculptured with a few coarse, subparallel 

 ridges, which are inflated where they cross the posterior 

 ridge; posterior ridge ordinarily well developed; hinge com- 

 plete, with strong teeth; hinge plate wide; berjk cavities deep 

 and compressed Quadrula Rafinesque . . 109 



Four sections: 109, no, in, 112. 



