92 THE CHICAGO ACADEMY OF SCIENCES. 
many small brown papille; superanal opening rather large 
papillose. (Simpson. ) 
Distribution: Mississippi Valley, Michigan to Louisiana. 
Geological distribution: Pleistocene. 
Habitat: Inrivers, ona muddy bottom. 
Remarks: Donactformis is easily distinguished by its zigzag 
epidermis. The males and females differ somewhat and have 
been described as distinct species. Inthe female the anterior 
end is rounded, projects beyond the umbones, which are ele- 
vated; the posterior end is short, truncated or obtusely biangu- 
lar and considerably inflated. Inthe male the anterior end is 
short, while the posterior end is long and pointed, and some- 
what compressed. It does not seem to be a common species, 
and but few specimens have been collected in the present area, 
where it is confined to the southern region. 
25. Plagiola elegans Lea, pl. xxi., fig]. 
Unio elegans Lea, Trans. Amer. Phil. Soc., Vol. IV., p. 83, pl. ix., fig. 138, 
1830-1. 
Unio truncatus RAFINESQUE, Say in American Conchology, p. 67. 
Shell: Trigonal, somewhat inflated, thick and solid, 
rounded before and obtusely biangulate behind; dorsal and 
ventral margins curved; surface smooth and shining, growth 
lines not prominent; umbones elevated, more or less inflated, 
yellowish or greenish in color, marked by several undulating, 
elevated ridges, generally eroded; anterior umbonal slope 
rounded, heart shaped when the valves are closed; posterior 
slope flat or excavated, separated from the sides of the shell by 
a strong ridge or shoulder; ligament short, wide, stout, dark 
brown or chestnut color; epidermis light yellowish green, with 
numerous dark green, zigzag rays, which are finer and smaller 
than in donaciformis (sometimes the rays are plain); cardinal 
teeth double in both valves, those in the left valve nearly equal, 
elevated, pyramidal or triangular, stout, serrated; those in the 
right valve unequal, the anterior being but a thickening of the 
hinge line, the posterior elevated, pyramidal, thick, striated; 
lateral teeth short, thin, lamelliform, serrated, curved ventrally; 
that in the right valve has a depression as in donaciformis,; ante- 
rior adductor muscle scar squarish, deeply excavated, striated; 
posterior adductor muscle scar ovate, lightly impressed, conflu- 
ent; protractor pedis muscle scar’small, wider than long, deeply 
impressed and striated; dorsal muscle scars as in donactformts ; 
pallial line impressed; cavity of the beaks of good depth; nacre 
