90 THE CHICAGO ACADEMY OF SCIENCES. 
impressed, crenulated; cavity of the beaks shallow; nacre sil- 
very white, iridescent on the posterior part. The anterior part 
of the shell is much thicker than the posterior part. 
Length, 49.50; height, 44.50; breadth, 29.50 mill. (1756). 
y A200: 46.00; Js 30:00 3“ (1756). 
Animal: Anal opening very small, apparently having small 
crenulations; branchial opening small with small thick set 
papille; branchie small, round below, inner the larger, free 
nearly the whole length of the abdominal sac; marsupium 
consisting of from four to six elongated ovisacs just behind the 
center of the outer gill. (Simpson. ) 
Distribution: ‘Western New York west to Arkansas, Mich- 
igan south to Texas. 
Geological distribution: Pleistocene. 
Hfabitat: Found in rivers, in afew feet of water on a muddy 
bottom. 
Remarks: Reflexa is a very distinct species, not to be con- 
founded with any other form. It varies very much in its orna- 
mentation from plain to strongly rayed, but the nodules seem to 
be very constant. Itis a rare and very restricted species, at 
present only known from the Calumet River. The specimens 
from this locality are yellowish with fine black rays made up of 
small dashes of color placed longitudinally. 
Genus PLAGIOLA (Rafinesque) Agassiz, 1852. 
Shell: WRather solid, somewhat triangular in outline, pointed 
posteriorly and having a sharply defined posterior ridge covered 
generally with a smooth epidermis which is often beautifully 
painted with wavy or zigzagged lines and markings; beaks 
not prominent, the region often being compressed; beak 
sculpture very faint, consisting of a few irregular corrugations; 
hinge line curved with two cardinals (pseudo-cardinals) and 
two laterals (pseudo-laterals) in the left valve, and one promi- 
nent cardinal (pseudo-cardinal) with sometimes one or two 
smaller ones, and one prominent lateral (pseudo-lateral), and 
a smaller inner one in the right valve. The female shell is 
generally slightly more inflated and developed in the postbasal 
region than that of the male, but in some cases it is extremely 
difficult to separate the sexes by the shell characters. 
Animal: Branchial and anal openings provided with 
papille; posterior part of the outer gill of female used as a 
