Ortmann; Families \m> Genera of Najades. 



265 



The ctiicr gills alone are marsupial, and the placentae are rather 

 distinct. Glochidia (see Ortmann, l<)ii/<, pi. 89, lig. 5) of sin. ill size, 

 subovate, without hooks. Their length and height is about the same, 

 0. 16 nun. 



Color of sofl parts whitish, with foot and gills grayish, and the 

 margin ol the mantle black posteriorly. In other specimens the foot 



mp 



9 9a 



Fig. 9. Pleurobena clava (Lamarck). Male, from Sandy Creek, Sandylake, 



Mercer Co., Pa. (Cam. Mus., Xo. 61, 4,589.) Coll. June 27, 1910. 



Fig. ga. Left gills of sterile female, from Shenango River, Pulaski, Lawrence 



Co., Pa. (Cam. Mus., Xo. 61, 4,591.) Coll. Oct. 4, 1910. 



is pale orange, as are also the margins of the mantle and adductors. 

 The gills are grayish brown. There are all intergrades between these 

 extremes. The placentae are white, cream-color, or pale orange. 



Pleurobema decisum (Lea). 



According to the similarity of the shell, and the notes on the anatomy 

 furnished by Lea (Obs., X, 1863, p. 405), this species belongs here. 



Genus Elliptio Rafinesque. 1819. 

 Simpson, 1900&, p. 700 (as section). 

 Shell more or less elongated, with straight longitudinal axis, not 

 oblique. Beaks not very near the anterior end, not very prominent, 

 with shallow beak-cavities; hinge-teeth well developed. Outer surface 

 without sculpture. 10 Epidermis generally rather dark, brown to 

 black, without rays, or with indistinct rays, chiefly so when young. 

 Rays straight, simple, and fine. Beak-sculpture tending to become 



H If U. spinosus belongs here, the diagnosis should be modified in this par- 

 ticular. 



