ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 315 
mantle were entirely connected as far as the posterior dorsal end. Of course the 
outer edges of the mantle remain separated, and if only superficially investigated, 
the appearance of a slit-like supra-anal, identical to that of other forms, is imitated. 
3. The edge of the mantle in front of the branchial opening. 
Anterior to the branchial opening the margin of the mantle is originally with- 
out any special differentiation. The two edges are about parallel and close to- 
gether to the anterior end of the animal, and do not possess any appendages in 
the shape of papille and the like. At most the inner edge is slightly crenulated, 
scalloped, or wrinkled, and the latter appearance often depends on the state of 
preservation (it becomes wrinkled by strong contraction in alcohol). The com- 
monest condition is when the papille of the branchial opening suddenly become 
small, assume the shape of crenulations, which soon disappear, and thenceforward 
the inner edge appears more or less smooth. The margin of the mantle (the part 
under both edges), which is thickened at the branchial opening, thins out gradually, 
and the thickening is hardly any longer noticeable at a short distance in front of 
the branchial opening. 
This condition is seen in all species investigated belonging to the following 
genera of Simpson: Quadrula, Pleurobema, Unio, Tritogonia, Alasmidonta, Stro- 
phitus, Symphynota, Anodontoides, Anodonta, and Ptychobranchus. 
In Margaritana Lea (Observ. VII, Pl. 29, fig. 104) has figured strong, papill- 
like crenulations on the edge of the mantle for a considerable distance in front of 
the anal opening. He does not give a detailed description of this structure in 
the text. In the specimens in my possession I find that the inner edge of the 
mantle in this region is strongly wrinkled and crenulated, the crenulations passing 
posteriorly into strong papille, such as are usually found at the branchial opening. 
I do not regard this as a special differentiation of structure, but it rather appears 
to me to be a forward continuation of the papille of the branchial opening; so 
that we should in this case regard the branchial opening as less distinctly defined 
anteriorly. In fact I cannot discern any sharp demarcation of the latter, such 
as is seen in other species. The branchial opening in Margaritana is ill defined, 
and passes gradually into the anterior margin of the mantle, a condition which 
again emphasizes the primitive character of this genus. 
There remain a number of genera, which are characterized by the peculiar 
structure of the marsupium, and in which it is restricted to the posterior part of 
the outer gills, and projects beyond the margin of the latter. These are the genera 
Obliquaria, Plagiola, Obovaria, Lampsilis, Micromya, and Truncilla. Among these 
we meet with interesting special structures on the edge of the mantle. 
