330 MEMOIRS OF THE CARNEGIE MUSEUM 
ascertained by the septa of the gills, and that we are able to make out the character 
of the marsupium also from the sterile female, it is now possible to properly place 
all species of which females are at hand. Thus I have ascertained, that the 
following forms of Simpson are true Quadrulas: Q. subrotunda, kirtlandiana, rubi- 
ginosa, trigona, pustulosa, metanevra, cylindrica, undulata, plicata hippopea, and 
Tritogonia tuberculata. Q. lachrymosa also belongs here, judging from specimens 
obtained in Kansas and from the middle Ohio River. 
Of other forms of Simpson’s Quadrula which I have investigated the following 
proved to have only the outer gills built to receive eggs: Q. tuberculata, coccinea, 
pyramidata, obliqua, cooperiana. With the exception of the first these all fall 
under Pleurobema, in Simpson’s sense, but it is somewhat different in the case of 
Q. tuberculata. Here we have additional characters, which have already been 
pointed out by Simpson, and which induced him to place this species in a separate 
subgenus, Rotundaria. The chief features are the absence of a supra-anal opening 
and the peculiar sculpture of the beak. The structure of the hinge and the color 
of the nacre are also rather peculiar, and there are so many distinctive characters 
in evidence, that I feel fully justified in elevating the subgenus Rotundaria to the 
rank of a genus. 
All other forms with only the outer gills serving as marsupium should belong 
to the two genera Plewrobema and Unio. There are no differences whatever in 
their soft parts, and these two genera can be distinguished only by the characters 
of the shell. Looking over Simpson’s elaborate diagnoses (they are rather de- 
scriptions), it is hard to find well-marked differential characters. The shape of 
the shell may be taken as the most important criterion. In Plewrobema it is quite 
solid, triangular, rhomboidal, or subovate, more or less distinctly oblique, that is 
to say, with the two axes giving the two principal dimensions (height and length) 
forming a distinct and sharp angle with each other. This causes the umbonal 
region, which is more or less elevated, to assume an anterior location in the shell. 
In Unio on the contrary the shell varies from oval to elongate, is straight, with the 
axes very nearly vertical to each other, with the result that the umbonal region is 
less elevated and less anterior. Further, in Pleurobema the color of the epidermis 
is from dark brown to light brown or olive, with prevalence of the lighter shades, 
and the color of the nacre is white or silvery, rarely lighter or darker pinkish or 
salmon-red, but never purple. In Unio, the epidermis is brown to black, very 
rarely lighter, with prevalence of the darker shades, and the nacre may be white, 
but more often assumes red to purple tints.!8 
18As will be shown elsewhere, the North American genus Unio should be separated from the typical European Unio 
(Type: pictorum Linnzus). For the former the name Elliptio Rafinesque 1819 is available. 
