810 TETRACENE 
“This shell is remotely, if at all, related to the other shells 
above mentioned (cicatricosus Say and varicosus Lea). There 
is apparently a slight relationship to Unio propinquus in its 
general facies. but the species is in reality quite distinct.” 
Doubtful and indeterminate species. 
PLEUROBEMA ABACUS (Haldeman). 
“General form of U. subplanus Conrad. Substance of the 
shell thick; umbones approximate, depressed, anterior end 
without interior cavity; posterior slope regularly arched; mus- 
cular and pallial impressions very well marked; epidermis 
brown, rough. 
Length 3.8, height 2.0, thickness 1.0 inch. 
Hab. Holston River, Tennessee.” (Haldeman). 
Unio abacus HatpEMAN, Jl. Ac. N. Sci. Phila., VIII, 1842, p. 
202. 
ee) 
Margaron (Unio) abacus Lua, Syn., 1852, p. 24; 1870, p. 38- 
Pleurobema abacus SIMPSON, Syn., 1900, p. 749. 
I! am wholly unable to say what this is as it never has been 
figured and I have never seen the type. Shells that are in the 
Lea collection and others that I have seen labeled Unio abacus 
Haldeman are much like Plewrobema appressum and cannot 
be the genuine abacus of Haldeman, which according to his 
measurements is a longer, larger, form. 
The following species are described by Rafinesque, but I am 
unable to make them out. 
Pleurobema mvytiloides RAFINESQUE, Ann. Gen. Sci. Brux., V, 
1820, Pp. 313, pl LX XXII, figs. 3-10. 
Pleurobema cineata RAFINESQUE, Ann. Gen. Sci. Brux., V, 
1820; p. 313: 
TETRAGENE AL. 
Male and female shells alike, solid; beak sculpture consist- 
ing of coarse, subparallel ridges; beak cavities deep; marsup- 
ium filling all four gills, smooth, pad-like. 
