749 PLEUROBEMA 
Unio law. Lea, Pr. Ac. N.eSct. Philagciy i879, 4.160; jl. Ane 
N. Sei. Phila.: VII, 1874, 2p. 8) pl a ietc4 Obs er 
1O7A,) Da.T2 pli. fie eA 
Umo pattinoides Lea, Pr. Ac. N. Sci. Phila., I, 1871, p. 193; 
Ji. Ac. N. Sei..:Phila., VW, 1874.69.16; soleing bee Once 
XIII, 1874, p. 20, pl. rv, fig. 12. 
Umno belluing Lea, Pr. Ac. No Sein Phila, 1) 1872.92 10% coe 
Ac. N. ok. Phila.; VIM 1874) p250; ai xvi, fig. 48; Ore 
XITI, 1874, p. 54, pl. xv, fig. 48. 
I have united under one name a number of nominal species, 
which, though they have some slight differences, do not seem 
to me to be worthy of specific or even varietal rank. In gen- 
eral this form is smaller than that, which I believe to be mac- 
ulatum, it is more inflated and its base line is more rounded. 
It is not so much lengthened as clava nor are its beaks placed 
so far forward as a rule. The type of Unio holstonensis is. 
quite a young, brightly rayed shell and very different from sev- 
eral larger, rough, rayless specimens that Dr. Lea has placed 
with it under that name, which I think, are his Unio pilaris. 
The group typified by Unio clava is one of the most puzzling 
in North America and I confess that in some cases I have not 
been able to satisfactorily untangle its intricacies. 
PI,EUROBEMA BOURNIANUM (Lea). 
Shell triangular, short, subinflated or inflated, solid ; beaks. 
very high, rather sharp, full, turned forward over a lunule, 
placed nearest to the anterior end, their sculpture a few broken 
ridges; anterior end somewhat obliquely truncated, posterior 
outline curved from the beaks to the base; base line curved in 
front, nearly straight behind; posterior ridge low, narrowly 
rounded, placed almost at the posterior outline, ending below 
in a point at the base of the shell; in front of the middle of 
the shell there is a wide, high, radial swelling and here the 
diameter is much the greatest ; between this and the posterior 
ridge there is a wide radial depression; surface with irregular 
growth lines; epidermis greenish-yellow or pale tawny, with 
conspicuous, narrow and wide, interrupted green rays, sub- 
