340 The American Naturalist. [April, 
solete, while a good series of shells will show every variation 
from those with no laterals at all, to others in which they are 
perfect. 
The latter species has been placed by some authors in Unio 
and by others in Margaritana. In Unio decumbens and hembelii 
there is a somewhat better development of laterals, though 
they are often not quite perfect, while in U. crassus and laosen- 
sis both cardinals and laterals are like those of ordinary 
Unios. Here then in a group of Naiads, which at least by the 
shell characters appear to be closely related, we have every 
variation from species which occasionally have neither cardi- 
nals or laterals to those in which they are perfect. 
So far as is known the animals of these different species do 
not greatly differ, and Lea’s description of that of Margaritana 
margaritifera would almost exactly answer for that of Unio 
monodontus. ; 
Margaritana confragosa, a species found sparingly through- 
out a considerable part of the Mississippi drainage basin can- 
not be referred, I think, to any group of Unios, but it evidently 
has a much closer relation to the Asperrimus and Plicatus 
groups than to any Margaritanas. This relation is shown in 
the form of the shell, which is like that of the species of both 
of these groups, and by its sculpture, there being two rows of 
tubercles radiating from the beaks after the manner of those 
of Unio asperrimus, and the body of the shell being plicate as 
are all the members of the Plicate group. The hinge of this 
species seems to have become somewhat degenerated or weak- 
ened, as it is of unusually light structure for so heavy a shell, 
and such teeth as appear are generally somewhat compressed. 
In some specimens the posterior cardinal of the left valve is 
recurved and cut into serrations on its edge, and fits into a 
somewhat open space under the beak of the right valve, some- 
thing after the manner of the Pressus group. It will be found 
in occasional specimens of the Asperrimus group that this pos- 
terior cardinal though much heavier, is recurved and serrate 
on its edge, and that there is a partial corresponding break 
in the usually wide hinge plate of the right valve. But in a 
large series of M. confragosa almost every variation may be 
