THE MOLLUSCA 



995 



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

 pseudocardinals; laterals, when present, very obscure. Gills 

 without water tubes and with scattered interlamellar con- 

 nections, which in certain places form irregular diagonal rows. 



Family MARGARITANEDAE. 

 Only one genus Margaritana Schumacher. 



The typical species, M.margaritiferaL. (Fig. 14805X1), 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, sub triangular, or elongated; hinge edentulous 

 or with pseudocardinals only or with both pseudocardinals 

 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, 113, 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; beak cavities deep 



and compressed Quadrida Rafinesque . . 109 



Four sections: 109, no, HI, 112. 



