20 University of Michigan 
Pleurobema maculatum (Con.), Simpson, "14, p. 737 
Pleurobema conradi Vanatta, ’15, p. 559. 
Lexingtoma dolabelloides conradi (Van.), Ortmann, ’18, p. 546. 
Plewrobema conradi, Van., Walker, ’18¢, p. 172. 
The new name conradi has been properly introduced by 
Vanatta to take the place of the preoccupied maculatus Con. 
If the two forms are to be regarded as varieties of the same 
species, which is not determined here, dolabelloides as the 
oldest valid name is that of the main species; while conradi 
is that of the variety. The earlier name maculatus Con. must 
not be considered. | 
Genus PLEUROBEMA Rafinesque, 1819 
Type: Pleurobema mytiloides Raf., ’20—Unio clava 
Lam., “10. 
Pleurobema Rafinesque, ’10, p. 427; ’20, p. 313 (no type named). 
Hermannsen, ’47, p. 292 (type, Umo mytiloides Raf.= Unio clava 
Lam.) ; Agassiz, ’52, p. 49; Simpson, ’00¢, p. 745, and ’14, p. 732. 
Scalenaria Rafinesque, ’20, p. 309 (no type named) ; Herrmannsen, 
"48, p. 422 (type, Umio scalenius Raf.). 
In his original description of Pleurobema (1819) Rafinesque 
cited two undescribed species, mytiloides and conica? The 
latter never was described, so that if the former can be rec- 
ognized, when subsequently described, it ipso facto became 
the type, the genus being monotypic. 
Pleurobema (1820) was .founded upon two species, myti- 
loides and cuneata; both are surely U. clava Lam. (which see). 
Herrmannsen designated U. mytiloides Raf., which was unnec- 
essary if our position that Plewrobema is really monotypic is 
correct. If not, his was the first designation of a type for the 
genus. Agassiz took up Rafinesque’s generic name, but with- 
out naming a type. Since, however, mytiloides (= clava) is 
the only species of Rafinesque included in the original diag- 
nosis, this becomes the type. Thus, Simpson was justified on 
