Ortmann: Families and Genera of Najades. :;•_".» 



Genus Plagioi.a Rafinesque. (1819.) 

 (Simpson, 1900&, p. 603.) 



Shell subtriangular, somewhat inflated, but peculiarly compressed 

 toward the beaks, with a distinct posterior ridge, and a narrow, trun- 

 cated, posterior slope. Disk not sculptured. Beak-sculpture indis- 

 tinct, consisting of a few, fine, concentric, and slightly and indistinctly 

 double-looped ridges. Epidermis yellow, greenish, or brownish, 

 painted with rays, which are broken into lunate, or squarish, blotches. 

 Male and female shells slightly different in shape, the female smaller, 

 more inflated, and slightly swollen in the post-basal region. 



Inner lamina of inner gills free for a greater or smaller distance. 

 Inner edge of mantle in front of branchial for a certain distance slightly 

 lamellar and with fine teeth, but without papillae in the female. Mar- 

 supium like that of Obovaria, kidney-shaped. Placentas lanceolate, 

 not very solid. Glochidia very large, spatulate, gaping at the anterior 

 and posterior margins. 



Type P. securis (Lea). 



In the soft parts, this genus stands essentially upon the same stage 

 of development as Obovaria and Amygdalonajas. Its chief character- 

 istics are the shape of the shell and of the glochidia. The latter are 

 quite unique, and possibly indicate a transition toward the glochidia of 

 Proptera. 



Plagiola securis (Lea). 



About half a dozen specimens from the Ohio and Allegheny in 

 western Pennsylvania have been collected by myself. I received from 

 1). Walker a gravid female from the Cumberland River in Kentucky, 

 and another from H. E. Wheeler from the Ouachita River in Arkansas. 



In September and October this species is regularly found gravid, 

 so that the beginning of the breeding season is normal. 



The soft parts have been described by Lea (Obs., X, 1863, p. 43). 



Anal and supra-anal separated by a manlle-connection of medium 

 length. Inner edge of anal crenulated, that of branchial with papillae. 

 In the female, the inner edge of the mantle in front of the branchial 

 is slightly lamellar, with fine teeth, which are rather distant, but with- 

 out papilla?. Posterior margins of palpi connected for about one- 

 fourth of their length. 



Gills and diaphragm of usual shape. Inner lamina of inner gills 

 more or less free. The maximum observed was free for about three- 



