ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 313 
the gills, which serve to draw the mantle together, although, in our species, the 
two sides of the latter never unite at this point (Compare fig. 4, on p. 289). The 
part above this point is the anal opening, which forms the outlet for the supra- 
branchial canals and the cloacal cavity (with the anus). Its upper (dorsal) bound- 
ary will be discussed below (see supra-anal opening). 
Below the diaphragm is the branchial opening, which leads into the branchial 
chamber, into which the gills hang down. Its lower or anterior boundary is 
marked by slightly projecting lobes of the inner edge of the mantle (often indicated 
only by a curve reaching from posterior to the lower margin), which approach 
each other, and beyond which the edges of the inner mantle touch each other, but 
without growing together. 
Anal and branchial openings generally possess on the inner edge peculiar crenu- 
lations, teeth, or papillae, which were already studied by Lea, who took special 
pains to describe them accurately, believing that they might be of systematic 
value. He lays great stress upon the presence or absence of papillze on the inner 
edge of the anal siphon. But according to my observations, this is not an important 
feature. There may be papille, which may be larger or smaller (always smaller 
than those of the branchial opening), and they pass gradually into fine teeth, 
scallops, or crenulations; or the edge may be practically smooth. Sometimes the 
teeth, or crenulations, appear different according to the state of preservation. 
In most eases crenulations (wrinkles or small folds), fine teeth, or very fine 
papilla are present. A smooth or almost smooth edge is found principally in the 
following forms: Quadrula metanevra, Q. cylindrica, Q. tuberculata (Rafinesque), 
Anodonta grandis, and A. imbecillis. In Anodonta cataracta I found crenulations 
of very variable character. On the other hand more distinct papillae appear in’ 
Pleurobema clava, Unio crassidens, U. gibbosus, U. complanatus, U. productus, and 
Anodontoides ferussacianus. 
The branchial opening always has papille, and they offer, although slightly 
varying in their development, no remarkable differentiations. They may be simpler 
or more complex (arborescent), and may be larger or smaller. 
2. The supra-anal opening. 
In most of our species, the upper part of the anal opening is separated from the 
anal by a complete union of the inner edges of the mantle, and is called the supra- 
anal opening (See fig. 4, on p. 289). This opening communicates under the united 
edges of the mantle with the anal opening, and its purpose is rather obscure. I 
am inclined to believe, that the connection of the edges of the two halves of the 
