posterior end of the foot; and a little bit farther back, the two outer canals (be- 
longing to the outer gills) open into and unite with this cloacal chamber.” 
In a posterior view (See fig. 4) the diaphragm separating 
the branchial and cloacal openings forms a rather short, sim- 
ple, horizontal bridge, from which the gills hang down. 
Sometimes there is a slight median incision, when the two in- 
| ner lamin of the inner gills are not fully united to their 
ends, but according to my investigations this has no system- 
atic value. 
We are to note an exception to the normal structure, and 
this is again Margaritana margaritifera. Here (See fig. 6) the op eae 
diaphragm represents an entirely different aspect, which is ice Sta meer 
due to the fact that, as we have seen above, the outer lam- Ye Showin branchial, i 
anal, and supra-anal open- 
ine of the outer gills are not connected with the mantle up to _ ings, diaphragm, and pos- 
their ends. Thus on the one hand the outer attachment of “""™*% = 
the diaphragm is not seen at the outer edge of the mantle, and the outer laminze | 
of the outer gills are free for a considerable distance, which causes the whole dia- 
ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 289 
Fie. 5. Anodonta grandis gigantea (Lea). Posterior end of Fia. 6. Margaritana margaritifera (L.). Posterior view, 
body, seen from below, with the diaphragm cut open in the showing branchial and anal openings, and the peculiar, in- 
median line, to show cloacal chamber.—o, openings of the su- complete diaphragm. No supra-anal is separated from the 4 
prabranchial canals of inner gills; 7, openings of the supra- anal. 
branchial canals of outer gills. 
| phragm to appear as if it were tapering to a median point; and, on the other 
; . . . 
hand, the cloacal opening (anal siphon) is much larger than usual. 
2This condition is not clearly represented in Brooks’ (1890, p. 275, fig. 143) diagrammatic figure. The openings of the 
suprabranchial canals of the outer gills into the common posterior cloacal cavity are not shown. Their position would be 
indicated by the two nervous strands starting from the parietosplanchnic ganglia and curving backward toward the posterior 
end of the gills. 
