8 University 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. VII, xl, 135). In addition, the color of the nacre is 

 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 E. 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. ravistellns 

 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. tabasco ensis "Charp." Kiister) 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). 

 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. 



