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. berlandieri Lea), Del- 
phinonaias (type U. delphinulus Morelet), and Phyllonaias 
(type U. paludosus Morelet). None of these are used and 
they are all included as synonyms of Actinonaias for the fol- 
lowing reasons: 
I. 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 te 
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) saller (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. 
