Ortmann; Families and Genera oi Najades. 273 



shorter than the supra-anal, but distinctly longer than the anal. Inner 

 edge "l anal with fine crenulations, that of branchial with papilla?. 

 Inner lamina of inner gills free, excepl anteriorly. Posterior margins 

 of palpi connected for about one-fourth of their length. Marsupium 



formed by the miter gills, ami of the usual structure. Color of soft 

 parts whitish. 



Genus Unio Retzius. (1788.) 

 Simpson, 1900b, p. 679 (restricted). 



Shell ovate, or more or less elongated, with straight longitudinal 

 axis, not oblique, and beaks not very close to the anterior end. Beaks 

 not \ri'\ prominent, with shallow beak-cavities. Hinge-teeth well- 

 developed. Outer surface without sculpture. Epidermis light or 

 dark, with, or without, rays. Beak-sculpture distinctly of the double- 

 looped type, or even zig-zag, with a distinct reentering angle of the 

 bar- in front of the posterior angle. Often the sculpture is rudi- 

 mentary, and consists of tubercles indicating the lower angles of the 

 original loops. 



Soft parts much like those of Pleurobema, Elliptic, and Uniomerus. 

 Mantle-connection between anal and supra-anal moderately long 

 (generally almost as long as the anal). Inner lamina of inner gills 

 free, except at anterior end. Marsupium formed by the outer gills, 

 with the usual structure (see Plate XVIII, figs. 4, 5). Gravid females 

 have not been seen by the writer, but the glochidia are described by 

 European authors as being moderately large, subtriangular, with a 

 hook on the ventral point of each valve. 



Type U. pictonun (Linmeus). 



This genus chiefly differs from the foregoing genera in the shape of 

 the glochidia and in the beak-sculpture. Although the marsupium 

 is similar to the North American genera Pleurobema, Elliptic/, and 

 Uniomerus, I do not think that this indicates close relationship, but 

 that it is due to parallelism of development. The genus Unio of the 

 Old World has started from certain Unioniiicc (with four gills serving 

 as marsupium) in an independent line of descent. We do not yet 

 know the forms which probably were ancestral to Unio. The shape 

 of the glochidium indicates that somewhere near Unio was the starting 

 point for the development of the subfamily AnoJoutincc. 



ZA? 



