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 M. 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 

 cannot be connected with it. In shell- 

 sculpture, M. hembeli is quite unique. 

 Its distribution (in southern Alabama 

 and Louisiana) offers a very interesting problem. 



Fig. 3a. Left gills of another 

 specimen from same locality. 



Family UXIOXID^. 

 Subfamily UNIONINjE. 



Simpson's (19006) North America genera: Quadrula, Tritogonia, 

 Pie it robe via, and Unio belong to this subfamily. Further, I have 

 shown (Ortmann, 1911c) 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 Tritogonia is simply a synonym 

 of Quadrula in Simpson's sense (see Sterki, 1907, p. 48, and Ortmann, 

 191 lb, 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 



