8 Umiversity of Michigan 
in A. medellina, to a certain degree, and a darkening of the 
epidermis appears to be a characteristic of many individuals 
of all species from the Medellin River. On the other hand, 
the block of teeth in the left valve (the individual teeth are 
indistinguishable) forms an almost equilateral triangle, and is 
set quite far anteriad and ventrad, as in A. medellina, while 
in Leptonaias they form an acute-angled triangle with the 
smallest angle posteriad, and set up more nearly under the 
beaks, as well represented in Lea’s figure of U. persulcatus 
(Obs. WEL xien25)9 ulin saddition. athe color ot the mackemis 
that of A. medellina and it comes from the same river. How- 
ever, the figure also very much resembles some of the more 
elongate, smooth forms of &. plexus (Con.), and the dimen- 
sions fit either species. For this reason, the best place for 
U. plicatulus, and the section Nephronaias with it, is under the 
synonomy of A. medellina (Lea), along with U. purpuriatus 
(Say), and sharing the same question-mark! Therefore, 
Leptonaias Crosse and Fischer (1894), type U. ravistellus 
Morelet, is used here as a subgenus of Elliptio, to include those 
southern Mexican and Central American forms, with the pecu- 
liar ornamentation, included in the genus Nephronaias as used 
by Frierson (1917). Coenonaias (type U. aeruginosus Mo.) 
and Simononaias (type U. tabascoensis “Charp.” Ktister) are 
synonyms. Both owe their origin to Crosse and Fischer (1894). 
Elliptio (Sphenonaias) plexus (Conrad) (1838). Plate I, 
figs. 4 and 5. 
Unio coloratus “Charp.” Kiister (1856). 
2 Unio plicatulus “Charp.” Kiister (1856). 
Unio pigerrimus (C. and F.) (1893). 
Three small specimens from the Rio San Juan (H, vii, c) 
closely resemble the smoother forms of this species. They 
represent, I believe, a depauperate race of the following form. 
