236. ANNALS OF THE CARNEGIE MUSEUM. 
cover any marked differences in the structure of the gills which 
might be due to sex. No gravid females were found. 
Color of soft parts brownish-white; 
foot grayish-brown, the darker part sud- 
denly marked off in a sharp line from 
the white abdominal sac; palpi and gills 
brownish, the latter more grayish pos- 
teriorly; mantle pale brown, its margin 
whitish with brown-black edge, most 
intense posteriorly; a black line on mantle 
separating anal and branchial cavities. 
This is a true Margaritana, much 
resembling in structure WM. margaritifera. 
But it has well-developed lateral hinge- 
teeth, and thus must be considered as a 
more primitive type. It has no closer 
relationship with M. monodonta, and 
iG) Ga Glefteills tof another) SCemMOn be connected with it. In shell- 
specimen from same locality. sculpture, M. hembeli is quite unique. 
Its distribution (in southern Alabama 
and Louisiana) offers a very interesting problem. 
Family UNIONID-. 
Subfamily UNIONINZ. 
Simpson's (1900b) North America genera: Quadrula, Tritogonia, 
Pleurobema, and Unio belong to this subfamily. Further, I have 
shown (Ortmann, r9IIc) that the European Unio also belongs here, 
as well as the Asiatic genera Parreysia and Lamellidens. I have 
further demonstrated, that the European Unio is not identical with 
the North American Unio, and that for the latter the generic name of 
Elliptio should be used. The genus Tyitogonia is simply a synonym 
of Quadrula in Simpson’s sense (see Sterki, 1907, p. 48, and Ortmann, 
I9IID, p. 329). 
In the structure of the soft parts there is not much differentiation 
in all these forms. The most important is that in some all four gills 
are used as marsupia (see Plate XVIII, fig. 1), in others only the two 
outer ones (see Plate XVIII, figs. 2, 4,5). The systematic value of this 
character has been doubted (see Frierson, 1909, p. 107). Yet I believe 
