316 MEMOIRS OF THE CARNEGIE MUSEUM 
As has been said, Ptychobranchus, which is also associated with these genera, 
has the normal structure. The same holds true of Obliquaria, Plagiola, Obovaria, 
and certain species of Simpson’s Lampsilis, namely: L. ligamentina, alata, and 
gracilis. But the beginning of a differentiation is here indicated. In all these 
the thickening of the margin of the mantle is more distinct, and extends farther 
forwards, sometimes as far as the middle of the ventral margin, where it disappears 
rather suddenly. Along this thickening we observe the inner edge of the mantle 
as a distinct keel, which is more or less crenulated or toothed. These teeth, which 
are the continuation of the papillae of the branchial opening, are also sometimes 
found for a short distance in normal cases, as described above; in the present 
genera, however, they extend farther forward, generally as far as the thickening 
of the margin of the mantle, and disappear beyond, or give way to mere crenula- 
tions. But in no case do these teeth assume the shape of papille, or appear as the main 
feature of this structure. They are rather elements accessory to the edge of the 
mantle itself, which is a narrow keel or lamella, projecting from the thickened 
margin of the mantle. This lamellar, projecting character is most evident in 
Lampsilis alata and gracilis. 
This structure of the edge of the mantle is present in both male and female, 
but it is generally more distinct in the female, chiefly so far as it refers to 
the thickening of the margin of the mantle and the size of the crenulations or 
teeth. 
While we may say that in these forms the structure of the margin of the mantle 
is practically normal, except that its thickening, and the crenulations of the inner 
edge are more emphasized; we find in the remaining species of Lampsilis, and in 
Micromya and Truncilla, other and more strongly pronounced differentiations, and 
there are two types observable within the genus Lampsilis (including Micromya), 
while in Truncilla peculiar conditions prevail. 
The first type is found in the following species: Lampsilis parva, iris, nasuta, 
recta, and Micromya fabalis. In these the thickening of the margin of the mantle 
is rather pronounced, and along it runs the inner lamelliform edge, which, however, 
is not simply crenulated or toothed, but possesses distinct, well developed, conical, 
more or less regular papille, which are rather sharply distinguished from the ir- 
regular, crowded, or massed papille of the branchial opening. These papillae run 
forward in various extent, and disappear rather suddenly; generally with the dis- 
appearance of the thickening of the margin, and give way to the normal crenulations 
or the normal smoothness of the anterior edge. In the different species, the 
papillz show the following peculiarities: 
