1895.] On the Validity of the Genus Margaritana. 339 
It is probable that when Dr. Lea described these two ani- 
mals, he never thought of their being closely related, yet, with 
a few trifling exceptions the description of one would answer 
for the other. In Margaritana complanata, which is a some- 
what solider, more rounded species, there is essentially the same 
arrangement of the teeth, the beak sculpture is exactly like 
that of the group, and specimens are occasionally found which 
approach Unio pressus in form. Unio charlottensis, a member 
of this section, is shaped much like Margaritana complanata, 
and the characters of the animal of the latter show that it is 
closely related to the other species of the group. 
In the section typified by Margaritana margaritifera, we have 
a set of Naiads all having elongated, usually arcuate shells, 
with black, rayless epidermis. They have commonly two 
more or less perfectly developed cardinals in the left valve, 
and one in the right; the hinge plate is elongated, narrow 
just behind the cardinals, but becoming heavier near the pos- 
terior end, and generally rounded on its inner face. All the 
species which I place in this group have a rather wide border 
of the prismatic outer layer of the shell:projecting beyond the 
nacre, and which is plainly visible from the inside, and all 
occasionally have dark-colored blotches on the nacre. Iin- 
clude in it Margaritana margaritifera Linn., having a circum- 
boreal distribution ; M. hildrethiana Lea, of the central Missis- 
sippi region ; Unio monodontus, found in the same territory as 
the last ; Unio decumbens Lea, of Tennessee and Northern Ala- 
bama ; Unio hembelii Con., of Louisiana; Unio crassus Retz., of 
Southern Europe, and U. laosensis Lea, of Southeastern Asia. 
In Margaritana hildrethiana, there are seldom any laterals, 
while M. margaritifera often has them more or less perfectly 
developed. A lot of the latter in the National Museum (Mu- 
seum No. 60,878) from the State of Washington, have a single, 
well-developed lateral in each valve, while two specimens in 
the Lea collection, one from Maine (Mus. No. 86,285) and 
another from Massachusetts (Mus. No. 86,286) have as perfect 
cardinals and laterals as any Unio. Unio monodontus usually 
has the cardinals more or less blurred, and sometimes in old 
specimens they are reduced to mere tubercles or are even ob- 
