PLEUROBEMA 807 
Margaron (Unio) wsopus Lea, Syn., 1852, p. 23; 1870, p. 34. 
Pleurobema @sopus SIMPSON, Syn., 1900, p. 764. 
Plethobasus «esopus ORTMANN, Ann. Car. Mus., VIII, 1912, 
p. 260, fig. 8. | 
Unio cyphia Conrap, New F. W. Shells, 1834, p. 68. 
Unio cyphius Say, Am. Conch., VI, 1834.—REEvE, Conch. 
Icon., XVI, 1864, pl. vit, fig: 28. 
Unio cyphias var. esopus Pavrer, Conch. Sam., III, 1890, p. 
150. 
Unio scyphius Kusrer, Conch. Cab. Unio, 1861, p. 181, pl. 
VLE il 
eeue: 
I am not positive as to the generic position of this and the 
following species, but I am inclined to place them in Pleuro- 
bema. No other living species of this genus has any develop- 
ment of tubercles, but a great number of fossil forms from the 
Tertiary of eastern Europe, which seem to be most like Pleuro- 
bema, are decidedly pustulous. The heavy, inflated, high beaks 
and the comiparatively shallow beak cavities are characters 
found in nearly all the species of this genus. In the single 
gravid specimen | examined (one cut of a large number from 
different localities) the outer gills were not filled very full of 
ova, though they were found throughout their extent, while 
the most careful search did not disclose any in the inner gills. 
In some specimens the radial ridge of nodules is strongly 
developed, so that the knobs become large, often compound 
humps. In this case there is a sort of wide radial depression 
behind the ridge and in front of the low posterior ridge. In 
other shells this ridge of nodules is feeble and there is no 
appreciable depression behind it. The species differs from 
cicatricosum in being more angular at the base of the nodulous 
ridge, and in having a harder, smoother epidermis. 
PLEUROREMA CICATRICOSUM (Sav). 
Shell subtriangular or subquadrate, somewhat inflated, solid, 
inequilateral ; beaks very high and full, turned slightly forward 
over a well-developed lunule; in the middle of the disk there 
is a curved, radial row of low, irregular nodules, reaching 
