ORTMANN: FAMILIES AND GENERA OF NAJADES. 265 
The outer gills alone are marsupial, and the placente are rather 
distinct. Glochidia (see Ortmann, 19110, pl. 89, fig. 5) of small size, 
subovate, without hooks. Their length and height is about the same, 
0.16 mm. 
Color of so{t parts whitish, with foot and gills grayish, and the 
margin of the mantle black posteriorly. In other specimens the foot 
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Fic. 9. Pleurobena clava (Lamarck). Male, from Sandy Creek, Sandylake, 
Mercer Co., Pa. (Carn. Mus., No. 61, 4,589.) Coll. June 27, 1910. 
Fic. 9a. Left gills of sterile female, from Shenango River, Pulaski, Lawrence 
Cot;, Pa. “(Gam Muss No: 6m, 4,501.)). ‘Coll. Oct. 4; roro: 
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 placenta 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 ELiipTio Rafinesque. I819. 
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.!® 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 
If U. spinosus belongs here, the diagnosis should be modified in this par- 
ticular. 
