322 MEMOIRS OF THE CARNEGIE MUSEUM 
posteriorly, in front of the branchial opening, widely separated from the outer edge, and 
approaches the latter again at about the middle of the ventral margin. In the space 
between the two edges, the margin of the mantle is thick, white, and spongy, and of 
a peculiar structure. Thus the inner edges cut off in part a separate chamber within 
the shell at the posterior lower end of the latter. Along the inner edge in this 
region are fine papillz, the largest behind, diminishing anteriorly, and becoming 
effaced before the edges come together again. Farther in front, the inner edge 
is smooth. In the male of this species, this peculiarity is not found. In this sex 
the two edges are almost parallel, and the papillee of the inner are extremely small. 
There is no doubt, that this structure also has something to do with the aération 
of the glochidia during the prolonged breeding season. But its particular action 
should be further studied. 
I have tried to find other features of the soft parts, which might offer dif- 
ferentiations available for the general systematic arrangement of the Najades of 
our region, but was not successful. Thus, for instance, as Lea has already pointed 
out, the palpi show certain differences, but these are rather slight, and of no sys- 
tematic value, and it is not necessary to give a detailed description of the latter. 
We are now ready, to discuss the general system of our Najades with reference 
to the characters described above. 
PART II. THE SYSTEM OF THE NORTH AMERICAN NAJADES. 
A. Drvistion Into FAMILIES AND SUBFAMILIES. 
In the course of the preceding investigation of the soft parts we took occasion 
to point out that Margaritana margaritifera is unique among our mussels. In 
fact some of the anatomical characters are so unusual, that this species appears 
as a stranger in our fauna, differing fundamentally from all other species. The 
most striking features are: (1) the absence of water-tubes in the gills; (2) the 
peculiar conformation of the diaphragm at the posterior end of the gills. There 
is not a single species among those which I have investigated, which shows even 
an approach to the structure seen in Margaritana, and the first one of these is 
something quite unexpected in so far that it does not conform at all to the type of 
gill-structure, as laid down in the text-books. 
In addition, the lack of a separate supra-anal opening and the incomplete 
demarcation of the anterior boundary of the branchial opening present additional 
peculiarities. Further, although I have not seen gravid females, and am un- 
acquainted with the marsupium and the larvee, these are known in the European 
