24 University of Michigan 



The group of Mexican forms with a tendency towards the 

 production of dorsal alae, which includes these species, may 

 be given any one of three names, all sections of Fischer and 

 Crosse (1894) : Cyrtonaias (type U. herlandierii Lea), Del- 

 phinonaias (type U. delphiniilus Morelet), and Phyllonaias 

 (type U. palttdosiis Morelet). None of these are used and 

 they are all included as synonyms of Actinonaias for the fol- 

 lowing reasons : 



1. From the shell characters, they all appear to be more or 

 less closely related to Actinonaias, although very probably sub- 

 generically or generically distinct. 



2. Until definitely placed in the synonomy of Actinonaias, 

 anyone who desired a little more variety in the nomenclature 

 of the North American uniones might possibly have placed 

 that genus in the synonymy of any of them. 



3. The choice of name should be left until the anatomy of 

 one of the types is thoroughly known. They are all prior to 

 Paraptera Ortmann (1911). 



Plagiola (Artonaias) opacata (Crosse and Fischer) (1893). 

 — Plate VI, fig. 35 ; plate VII, figs. 27-38. Fifty-eight speci- 

 mens from Lake Catemaco (H, vii, d), about one-half mile 

 from the outlet in the Rio San Juan River system. Although 

 no soft parts were obtained, the marked dimorphism of this 

 lot of specimens presents convincing evidence that the type of 

 U. opacata C. and F. is a female of a species closely related to 

 Plagiola (Artonaias) sallei (C. and F.) (1893). 



Checkerboard graphs, showing the variation in length and 

 the height-index, the variation in length and diameter-index, 

 and the variation in the two indices, were made for all 58 

 specimens, and gave bimodal arrangements in each case. Of 

 the photographs presented (plate VII), the left hand column 

 and the two central figures are plainly of the female type (figs. 



