THE NATURAL HISTORY SURVEY. 69 
ened with a distinct cardinal tooth in each valve, anterior to the 
apex of the umbo; muscle scars and pallial line distinct but 
not impressed; dorsal muscle scars situated on the anterior face 
of the cavity of the beaks, deep and very distinct; cavity of the 
beaks rather deep; nacre bluish white, iridescent; in old spec- 
imens the interior of the shell is tinged with salmon. . 
Length, 69.00; height, 87.00; breadth, 28.00 mill. (coll. Jensen). 
Ss 49.00; ie 28.00; a 20.00 '* (12946). 
Animal: Not differing essentially from S. edentulus. 
Distribution: Mississippi and Ohio valleys; Great Lakes. 
Geological distribution: Pleistocene. 
Habitat: Similar to that of S. edentulus. 
Remarks: This species may be distinguished from edentulus 
(which it greatly resembles, and of which it may be but a color 
variety) by the following points: The umbones are more 
inflated, and are situated nearer the center of the shell; the sculp- 
ture of the beaks is finer. and less conspicuous; the ground color 
of the shell is greenish andthe posterior end is acutely rounded. 
The cardinal tooth is also much more developed. In the young, 
the rays sometimes form broad bands of green. It is not at 
all impossible for the species to ultimately become a synonym 
of S. edentudus, as it is quite difficult to distinguish some forms 
of the two species. Pavonius is not common in the area under 
discussion, 
Genus UNIO Relzius, 1788. 
Shell: Oval to elongated, rather solid, generally having a 
more or less developed posterior ridge and becoming arcuate 
with age; epidermis mostly dull colored and soft, rarely rayed; 
beak sculpture consisting of a few coarse ridges, nearly parallel 
with the growth lines, sometimes a tendency toward being 
double looped or to break into nodules. Cardinals (pseudo- 
cardinals), two in the left valve and one (sometimes a small 
second tooth) in the right valve, laterals, two in the left and 
one (sometimes a small second tooth) in the right valve, 
laterals sometimes blurred or undeveloped, cavity of the beaks 
shallow. 
Animal: Much like that of Anodonta, but generally having 
papille more or less developed on the anal opening. Embryos 
filling the entire outer gill of the female and forming a smooth, 
even pad, not so swollen as in Anodonta. 
