PLEUROBEMA 735 
Generally raved, 
Rays broken. 
Compressed. planius, favosum. 
Inflated. ornatum, acuens, cuneolus, appressum, 
clinchense, Iesleyt. 
Rays entire, or nearly so, 
Compressed. bigbyanum, pudicum, oviforme. 
Subinflated to inflated. validum, barnesianum. 
Group of Pleurobema clava. 
Shell solid, triangular; beaks high, generally anterior; beak 
sculpture consisting of three or four broken, coarse, irregular 
ridges; epidermis yellowish or tawny, marked with broken 
green rays, which show a tendency to form square spots; 
pseudocardinals often somewhat lengthened and more or less 
parallel with the laterals. 
Animal having the inner gills the larger except at the ex- 
treme posterior end, free nearly or quite their whole length 
from the abdominal sac; marsupium occupying the entire outer 
gills; branchial opening rather large, with small papilla; anal 
opening with minute papillee or crenulations. Animal dirty 
whitish to salmon. 
PLEUROBEMA CLAVA (Lamarck ). 
Shell decidedly triangular, more or less inflated, solid ; beaks 
excessively high, full, placed almost at or even in front-of the 
anterior end, turned forward over a decided lunule that ex- 
tends under and behind them, their sculpture consisting of a 
few strong, irregular, often broken, ridges that turn up be- 
hind ; anterior end generally truncate, often with a slope down- 
ward and backward; sometimes the truncation is square, at 
other times this end is evenly rounded and projects a little in 
front of the beaks; base line nearly straight or somewhat 
curved; dorsal line curved, the curve being sharper at or a 
little behind ihe middle and meeting the base line at a point 
on or a very little above the base; posterior ridge rather low, 
but well defined, narrowly rounded, curved up in the middle, 
