22 University of Michigan 
system, is a small-stream form of the same thing, mainly 
described from males and from rather immature females that 
had not yet developed the characteristic shape of the older 
specimens. The epidermis, hinge armature, obliquity of the 
prismatic layer and nacre of the two forms are practically 
identical, except that typical L. fimbriata plainly shows its 
exposure to a more severe environment. A youngish shell, 
approaching L.fimbriata, in the A. N. S. P. from “near Tam- 
pico,” perhaps represents the male of typical A. disca (Lea). 
Some of the young shells of disca in the A. N. S. P. are indis- 
tinguishable from some specimens of L. fimbriata, which might 
be regarded as females that had not yet completely developed 
the adult dimorphism. ) 
Ortmann (1912) has already shown that the marsupial char- 
acters of fimbriata are those of the general Plagiola-Paraptera- 
Actinonaias type. If the hypothesis in regard to the sexual 
dimorphism of A. discus is correct, the section Disconaias C. 
and F. (1894) (type U. discus Lea) is more or less interme- 
diate in shell-characters between Actinonaias and Plagiola, as 
the males are more or less Lampsilis-like in shape, while the 
completely developed females bear considerable resemblance to 
typical Plagiola. The dimorphism of A. disca (Lea) is more 
marked than that of A. walkeri, as the females of the latter 
species do not differ so much in general shape from the males, 
but the shape of the post-dorsal swelling and beaks in the 
females is peculiar, and agrees with that of A. disca. 
The section Disconaias thus contains two species, one of © 
which may be divided into two subspecies: A. disca disca 
(Lea), A disca fimbriata (Frierson), both from the Panuco 
River system, and A. walkeri from the Rio San Juan. The 
dimensions given in the partial description of L. sapperi von 
Thering (1901) are similar to those of A. walkeri, and some 
