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318 MEMOIRS OF THE CARNEGIE MUSEUM 
more lamellar, and the papille form merely a fringe on this narrow lamella. In 
the male of this species, the papillae are extremely small. 
Lampsilis recta. The female is like that of L. nasuta, but the papille are 
much larger (sometimes with two tips), rather regular, and form a crowded, very 
distinet row, which stops quite suddenly at about the middle of the ventral margin. 
The largest papillae stand just at the point where they stop, but the difference in 
size is not very great, and the increase in size is slight in the postero-anterior 
direction. The strong development of the papilla makes the inner edge of the 
mantle, on which they stand, appear as distinctly projecting beyond the irregularly 
crenulated or smooth anterior part. In the male, the margin of the mantle is 
less thickened, and the papilla are much smaller, yet they are present. 
The second type of development of the inner edge of the mantle, found in the 
genus Lampsilis, is observed in the following species: L. luteola, radiata, orbiculata, 
ventricosa, ovata, multiradiata, and cariosa. Here the specialization is shown not 
by a development ‘of papillae, but the inner edge itself is greatly enlarged. It 
forms a rather broad, lamellar keel, which stands upon the thickened margin of the 
mantle, possesses great powers of expansion and retraction, and occupies a con- 
siderable distance from the branchial opening forwards, as far as to about the 
middle of the ventral margin. Here the lamella suddenly stops, but is prolonged 
anteriorly into a free, longer or shorter, ribbon-like flap, which also may be expanded 
to a considerable length and again retracted. 
This peculiar “flap’”’ has been noticeed by Lea in several species (radiata, luteola, 
ventricosa, ovata, multiradiata). He also has observed that there is, in some forms 
on the posterior end of the lamellar expansion, right in front of the branchial 
opening, a very curious “eye spot” (dark, round spot, upon a light background). 
Simpson mentions this character for his section Lampsilis of the genus Lamp- 
silis, and says also that in the section Hurynia it is “sometimes” present. All of 
the species of the first section known to me possess it, but not all of the second 
one. Apparently Simpson did not attribute much value to this feature, and he 
says that it is found only in the female, and during the breeding season. According 
to my experience, however, it is a regular character of all species, where it is ob- 
served; and is found not only in the female, but also in the male, although in the 
latter only in a rudimentary condition. I have been able to make it out in all 
males investigated. I consider it a very important systematic character, which 
is connected with the aération of the embryos in the marsupium, and consequently 
developed at its best in the female during the breeding season, but it is permanently 
present. Probably also in the other species of Lampsilis described above the cor- 
responding papillz have the same physiological function. 
