Ortmann: Families vnd Genera oi Najades. 



287 



Supra-anal and anal openings widely separated, the one about as 

 long as the other, but the united \><i\\ of the margin of the mantle 



sa 



Fk;. 13. Anodonta cygnea (Linnaeus). Male, from Obra South Canal, Sepno, 

 Prov. Posen, Germany. (Cam. Mus., No. 61, 4,956.) 



mp 



between them longer than either, with slight variations in length. 

 Anal with crenulations, branchial with papillae. Palpi with the pos- 

 terior margins united for one- 

 fourth or one-third of their length. 

 Gills and diaphragm as usual, inner 

 lamina of inner gills free, except 

 at anterior end. 



In the male, the septa of both 

 gills are rather distant; in the sterile 

 female the septa of the outer gill 

 are very crowded, forming very 

 narrow water-tubes (see Plate 

 XVIII, fig. 7). The whole outer 

 gill is marsupial, and at its edge 

 there is heavy tissue which permits 

 the distending of the gill when 

 charged. Within the marsupium 

 each water-tube is divided, in the 

 breeding season, into three tubes, 

 two narrow lateral (secondary) 

 water-tubes lying toward the faces of the gill, and a central larger 



Fig. 13a. Left gills of a sterile 

 female, from Mogelnitza River, Prov. 

 Posen, Germany. (Cam. Mus., No. 

 61. 4.953-) 



