ORTMANN: FAMILIES AND GENERA OF NAJADES. ihe 
shorter than the supra-anal, but distinctly longer than the anal. Inner 
edge of anal with fine crenulations, that of branchial with papille. 
Inner lamina of inner gills free, except anteriorly. Posterior margins 
of palpi connected for about one-fourth of their length. Marsupium 
formed by the outer gills, and of the usual structure. Color of soft 
parts whitish. 
Genus Unio Retzius. (1788.) 
Simpson, 1900), 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 very 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 reéntering angle of the 
bars 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, Elliptio, 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. pictorum (Linneus). 
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, Elliptio, and 
Uniomerus, | 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 Unionine (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 Anodontine. 
