658 UNIO 
planatus had been used for this species by Lister and Solander 
long before Say’s name was applied I think that the preference 
should be given to the well-known name of Solander. 
The above description is intended merely to cover the more 
typical manifestations of this abundant and protean form. The 
shells are often larger than the largest dimensions given, they 
are sometimes smaller than the smallest measurement, they 
may be more compressed or inflated in proportion to the length, 
they may vary in form so that occasional specimens are al- 
most evenly elliptical. In many cases the greatest diameter of 
the shell is at the posterior ridge and it is wedge-shaped in 
front of this; other specimens are regularly lenticular when 
viewed from above. Dr. Lea and a number of other American 
conchologists agreed on uniting under the name of Unio com- 
planatus nearly all the forms of this group found north of 
about the latitude of Washington. South of that they began 
to apply specific names to the different variations, most of 
which were no more divergent than the northern mutations. 
When this was once begun there seemed to be no place to stop 
for no two lots of shells of this exceedingly puzzling group 
just agreed. ‘The result was a great multiplication of names 
and almost endless confusion. I believe that the group of 
which Unio complanatus may be taken as the type is one of 
the most difficult to satisfactorily arrange of any of the 
Naiades. I have gone over a large proportion of the types 
and great quantities of material again and again in the en- 
deavor, not to name up all the specimens before me, for that 
would be impossible, but to draw some kind of specific lines 
by which a majority of the material might be named and I have 
found it well nigh a hopeless task. I have given varietal rank 
to a few of the forms, which have somewhat marked and con- 
stant characters. 
Var. jejunus Lea. 
Shell much compressed; posterior ridge widely doubie, end- 
ing behind near the base in a wide biangulation; epidermis 
