Ortmann; Families and Genera of Najades. ."»<)5 



c\. Shell rounded, ovate, or 9ubelliptical, without or with indistinct posterior 

 ridge. Glochidia oi normal size and shape, 3ubovate. 

 1/1. Shell rounded or ovate, swollen. Epidermis brownish rarely greenish, 



with indistinct rays Obovaria. 



(/:. Shell subovate or subelliptical, compressed, 01 only slightly swollen. 

 Epidermis greenish or yellowish, with more or less distinct rays. 



Nephronajas. 

 c. Shell ovate, triangular, swollen, or suhelliptical and compressed. Glo- 

 chidia of abnormal size or shape. 

 d\. Shell subovate or subtriangular, with a strong posterior ridge. 



oi. Glochidia of normal shape, but abnormally small size. Shell 



subovate or elongate Amygdalonajas. 



.•;. Glochidia spatulate, with gaping margins, large. Shell sub- 

 triangular Plagiola. 



di. Shell subovate or subelliptical, more or less compressed, often winged, 

 without distinct posterior ridge. 

 ci. Glochidia of normal shape, but abnormally small. Shell rather 



thin Paraptera. 



a. Glochidia celt-shaped, with two spines on each valve. Shell thin 



or rather thick Proptera. 



b : . Inner edge of mantle with papillae or flaps. Male and female shell distinctly, 

 and often greatly different in shape. 

 a. Inner edge of mantle parallel with and close to the outer edge. Shell 

 ovate, elliptical, or elongated. Glochidia subovate. 

 d\. Inner edge of mantle with papillae. 



ft. Shell with nodulous plications upon the posterior slope. 



Medionidus. 



e->. Shell without sculpture Eurynia. 



di. Inner edge of mantle forming a ribbon-like flap Lampsilis. 



ci. Inner edge of mantle in front of the branchial opening more or less remote 

 from outer edge in the female. Shell variable, not ovate or elliptical. 

 Glochidia almost semicircular Truncilla. 



In all these genera, we have a beak-sculpture, which is rather rudi- 

 mentary, and, when developed, either of the concentric or the double- 

 looped type. Beak-sculpture in this subfamily is apparently a char- 

 acter becoming more or less obliterated, and thus cannot be used for 

 general systematic purposes, although it is available as a subsidiary 

 character in a few cases. 



Genus Ptychobranchus Simpson. (1900.) 

 (Simpson, 19006, p. 612.) 

 Shell subelliptical, somewhat elongated. Disk smooth, sometimes 

 with ridges on the posterior slope. Beak-sculpture indistinct, con- 

 sisting of a few ridges, the first concentric, the others slightly double- 



