ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 321 
system, but I think, according to the above observation, that it has no closer 
relationship with this species. The latter has no trace of a flap, in fact, it hardly 
shows any differentiation on the edge of the mantle. Lampsilis ligamentina is no 
Lampsilis at all. It is true, the males of these two species are very similar; but 
this is not true of the females. The peculiar shape of the female shell of L. orbiculata 
is quite remarkable, and entirely unlike that of the female L. ligamentina. 
Lampsilis luteola and radiata possess a similar flap, and are practically identical 
with each other in this respect. Here the edge of the ribbon possesses numerous, 
irregular teeth, increasing in size toward the anterior free lobe; and further, there 
are distinct teeth on the inner edge of the mantle in front of the free lobe, di- 
minishing in size anteriorly till the edge becomes almost smooth. In the male, 
the anterior end of the flap is indicated by large teeth, which gradually pass into 
the scalloped and smooth anterior edge, and again all parts are smaller in the 
male, and less typically developed. On the inside of the flap and lamella there is 
a broad band of black color, and there is also an eye spot posteriorly, but this is 
hard to see in contracted alcoholic material. 
There remains the genus Truncilla to be discussed. In T. triquetra we see 
conditions, which somewhat approach the first group of Lampsilis, where there 
are papillee in front of the branchial opening. The swelling of the margin of the 
mantle is distinct, but does not extend far forward, and the inner edge along the 
swelling possesses a number (four to six) of conical papillae, which are not very 
large, and are followed farther forward by a number of still smaller papille di- 
‘minishing in size. In front of these the edge is smooth. But in this species we 
see a feature in the female, which, although not very striking, distinctly indicates 
the beginning of the most prominent peculiarity of T. perplexa rangiana, to be 
discussed below. The inner edge of the mantle, where it carries the papille, 7s 
not quite close and parallel to the outer edge, but runs at a certain distance from the 
latter, and the space between the two edges has a peculiar spongy structure. This 
spongy structure is also developed to a certain degree in the species of Lampsilis, 
but it is not so markedly restricted to the posterior part of the margin, just in 
front of the branchial opening. In the male of 7. triquetra the two edges are 
almost parallel and close together, and the papilla of the inner edge are rather 
small. While in the female T. triquetra the separation of the inner edge from 
the outer is barely indicated, this condition furnishes the most striking feature in 
T. perplexa rangiana. Lea already described it (Observ. X), and Simpson (1900, p. 
516) mentions it in the generic diagnosis, but without having properly understood 
this structure. In the female of 7. perplexa rangiana, the inner edge of the mantle is 
