P1LEUROBEMA 747 
line slightly curved; anterior end rounded; outline of the dor- 
sal slope curved, raised almost to an angle in the middle; sur- 
face nearly smooth; epidermis greenish-tawny with a few con- 
spicuous, widely interrupted rays, the whole rather bright: 
left valve with two ragged pseudocardinals, the anterior small, 
and two nearly straight laterals; right valve with three pseudo- 
cardinals, the median one large, the others small, and a slightly 
double lateral; muscle scars small, impressed; beak cavities 
shallow, compressed; nacre silvery, thick in front, thin, cop- 
pery, iridescent and almost transparent behind. 
Length 27, height 22, diam. 12 mm. 
Type localitv, Alabama. 
Umo ornatus Lea, Pr. Ac. N. Sci. Phila., V, 1861, p. 4; Jl. Ac. 
Ne pei winnila.. Vi. 1S02.\p. O5, plexity fic! 2394; Obs; VITT, 
1862, p. 80, pl. x1, fig. 234. 
Margaron (Unio) ornatus LA, Syn., 1870, p. 57. 
Pleurobema ornata SIMPSON, Syn., 1900, p. 749. 
I have no doubt that the only specimen I have seen of this. 
the type, is a young shell, though Dr. Lea believed it to be 
nearly adult. It is one of those puzzling forms, which have 
no very decided characters, yet do not seem to belong to any 
other species. It is more rhomboid than oviforme, not so 
much drawn out behind to a point. It is much less inflated 
than tesserule, is smaller and less triangular than bigbyense, 
to which Dr. I,ea compares it. 
PIEUROBEMA APPRESSUM (T[,ea). 
Shell subrhomboid, very inequilateral, the beaks being placed 
almost at the extreme anterior end, solid, subcompressed or 
scarcely inflated, with a well-developed, rounded, curved pos- 
terior ridge. In front of this ridge the shell is somewhat flat- 
tended, the compressed area extending to a sort of rounded, 
radiating, wide ridge near the anterior end; beaks high, full, 
turned forward over a well-developed lunule, their sculpture 
not observed: surface with irregular growth lines; epidermis 
somewhat wrinkled, greenish-yellow in the young state, dark 
tawny-brown in the adult shell, with faint indications of broken 
