THE NATURAL HISTORY SURVEY. 81 
from the upper part of the shell toward the ventral part; umbones 
large, elevated, inflated, dark horn color, marked by about five 
fine, diverging wrinkles; anterior umbonal sloperounded, short; 
posterior slope long, obtusely angular, excavated; ligament 
long, wide, stout, very dark brown or horn color; epidermis 
black or reddish brown; cardinal teeth double in both valves, 
the anterior smallest in both valves, but that in the right 
valve much smaller; all depressed, stout, triangular, very 
deeply grooved and channeled; lateral teeth long, stout, 
thick, elevated, serrated, directed ventrally; connecting plate 
thick, wide, smooth; anterior adductor muscle scar_ kid- 
ney shaped, very deeply impressed, very strongly marked 
by elevated ridges; posterior adductor muscle scar truncated- 
oval, distinct but not much impressed, striated; protractor pedis 
muscle scar wider than long, very deeply excavated, striated and 
strongly pitted; dorsal musclescars large, very deeply impressed, 
one in each valve situated on the under side of the connecting 
bridge; pallial line crenulated, impressed; cavity of the beaks 
deep; nacre silvery white, pearly on the posterior portion. 
Length, 79.00; height, 64.00; breadth, 50.00 mill. (coll. Handwerk). 
- SOG sac GasO0 a 52,00 '' (138003). 
Animal; Analopening very large, only slightly crenulated; 
branchial opening very large with many small papille; palpi 
long and falcate, united above and behind; gills large, inner 
much the larger, free from the greater part of the abdominal 
sac; not gravid. (Simpson.) 
Distribution: Western New York to Arkansas and Iowa, 
Michigan to Alabama and Texas. 
Geological distribution: Pleistocene. 
Habitat: In the larger rivers, in rather deep water, generally 
on a muddy bottom. 
Remarks: Plicata is generally confounded with wxdulata 
Barnes, but there is no good reason for this, as they are quite dis- 
tinct and easily separated. P/icata is always strongly inflated 
at the umbones, which are much elevated, and the shell is gen- 
erally broader than undulata. The undulations are fewer in the 
present species and are differently arranged. In undulata the 
umbones are flat and depressed, and the whole shell is com- 
pressed. The cardinal teeth are proportionately heavier in the 
present species. It does not seem to be very common in this 
area, and is confined to the lower part of the Desplaines River. 
