230 Annals of the Carnegie Museum. 



to its systematic value, the beak-sculpture, when properly understood, 

 indicates certainly systematic affinity, but is not fit to be used for the 

 distinction of larger groups, since it is very likely that the different 

 types were developed rather early, and are found side by side among 

 the more primitive groups of Najadcs, the Unionincc for instance. 

 Yet in the more advanced groups often only one type is found. Thus, 

 for instance, among the Anodontince and Lampsilincr, we possess 

 only the first type up to the double-looped structure, while the zig-zag 

 structure is practically absent, and no trace of the other type (the 

 radial) is ever found. On the other hand, it seems that in the Mu- 

 telidce only the radial type is present, provided there is any sculpture 

 at all. In addition, conditions become yet more complex by the fact 

 th.u the beak-sculpture in general seems to be a character which is 

 subject to obliteration, and anywhere within the system we may expect 

 to meet forms which have reduced their beak-sculpture to a lesser 

 or greater degree, often to complete disappearance. Thus we may say, 

 in a general way, that beak-sculpture, although important and in- 

 dicating the minor affinities, is unfit to be used for the distinction of 

 the larger groups. 



Family MARGARITANID.F. 



I recognize only one genus in this family, to which a number of 

 species have been assigned by Simpson, of which, however, the 

 structure of four only is known. 



Genus Margaritana Schumacher, i Si 7 



Simpson, 1900b, p. 674. 



Margaritana margaritifera (Linnaeus). 



Some twenty specimens are at hand, from the drainage of the upper 

 Little Schuylkill River in Schuylkill County, Pennsylvania, collected 

 by myself; soft parts of another specimen from the Auma creek, near 

 Weida, Saxe-Weimar, Germany (drainage of Elster river) ; and three 

 complete specimens from the Perl-Bach at Postfelden, near Falken- 

 stein, Bavarian Forest, Germany. For these German specimens I am 

 indebted to Mr. W. Israel. 



Published figures: Photograph of soft parts in shell, by Cail (1910, 

 pi. 4, figs. A and B) ; of gills, by Ortmann (1911ft, p. 285, fig. 1, and 

 pi. 87, fig. 11). 



