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.iimhriata, 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 ^. disca (Lea) is more 

 marked than that of A. zmlkeri, 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. sap peri von 

 Ihering (1901) are similar to those of A. zvalkeri, and some 



