ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 323 
Margaritana through the studies of Harms (1907, pp. 817 and 818). The marsupium 
is formed by all four gills, and the glochidium is apparently of the Quadrula-type.® 
These characters we must regard as expressing a distinctly primitive type of de- 
velopment in the case of Margaritana. : 
All this is sufficient to impress upon Margaritana the stamp of oddity and 
singularity, which justifies us in saying that this form stands by itself in the system, 
and should not be associated with any other form. This is best expressed by 
creating a family for it (Margaritanide). 
Yet there is possibly another species, which may go with it, Margaritana 
monodonta (Say). The soft parts of this have been described by Lea (Observ. X), 
and as far as the description goes, we may say that it agrees with Margaritana 
margaritifera. Lea especially emphasizes the peculiar diaphragm and the absence 
of a defined supra-anal opening. 
Leaving the genus Margaritana aside and turning our attention to the rest 
of our Najades, we find that the most prominent differentiations are found in the 
sexual apparatus, and chiefly in the general form and the finer anatomical structure 
of the marsupium. This part of the anatomy has been used by Simpson for his 
system. But his classification only serves in part to bring out the natural affinities. 
Summing up what has been discussed above, we are led to distinguish three groups 
among our Unionide according to the structure of the gills. 
1. In some the water-tubes of all or of some gills serve, in the gravid female, 
as ovisacs (receptacula for the ova and embryos). Their only differentiation is 
in the development of the septa separating the ovisacs, which more closely approach 
each other, are stronger, and possess a folded epithelium, adapted to the swelling 
of the marsupial gill in the breeding season. There are no other modifications, 
and the edge of the marsupial gill is not changed, and always remains sharp. 
Tt goes without saying, that this is clearly the most primitive type of the three. 
There are no special adaptations to an extended breeding season, and the latter 
is short in all these forms,—they are tachytictic. This type is found in Simpson’s 
genera Unio, Pleurobema, Quadrula, and Tritogonia. 
I have given above the essential characters of the soft parts. Of less impor- 
tance are the following: marsupial gill little or only moderately swollen in the 
breeding season; marsupium either formed by all four gills, or only by the outer 
gills in their whole length; supra-anal opening only in rare instances not separated 
from the anal, generally well defined by the connection of the margins of the 
Harms (p. 820) describes “rather strong” hooks in the glochidium, but in his figure (p. 818, fig. 4) there are only 
mere traces of them, and they cannot be compared, by any means, with those of the Anodonta-glochidium. 
