THE NATURAL HISTORY SURVEY. 107 
teeth double in both valves, equal in the left and the anterior 
tooth very small in the right, thick, solid, triangular, deeply 
serrated; lateral teeth rather short, thick, solid, not much 
elevated, almost smooth, the lateralin the right valve very wide 
at the base and rapidly coming to a point; connecting plate 
very thin and narrow; anterior adductor muscle scar very 
deeply excavated, longer than wide, coarsely striated; posterior 
adductor muscle scar rounded, length and breadth equal, deeply 
impressed, striated; protractor pedis muscle scar wider than long, 
deeply excavated, striated; dorsal muscle scars situated on the 
posterior face of the cardinal teeth and on the wall of the cavity 
of the beaks, numerous, very deeply pitted; pallial line deeply 
impressed, especially on the anterior end; cavity of the beaks 
very shallow; nacre silvery white, iridescent, particularly on the 
posterior portion of the shell. 
Length, 82.00; height, 46.00; breadth, 28.50 mill. (9840). 
ee GOOF us 39.00; ae 22:00) (9840). 
ae 65.50; we 35.00; Be 20200 3 (12419). 
Animal: Generally flesh colored; anal opening small, with- 
out papille; branchial opening rather large, papillose; ctenidia 
not large, wide, rounded before, pointed behind, yellowish 
brown in color, united above for their whole length; labial palpi 
not large, rounded, triangular, yellowish white, united at base; 
siphons yellowish white inside, edged with a wide line of 
brownish black; mantle whitish or brownish, edged with black; 
liver dark brown; foot yellowish brown or dark flesh color; 
abdomen pearly white, flecked with pure white near the dorsal 
border. ‘The posterior part of the outer branchium is used as 
a marsupium. Heart regular, sixteen beats per minute. 
Distribution: Mississippi Valley. 
Geological distribution: Pleistocene. 
Habitat: Ponds and rivers, buried in black, slimy mud toa 
depth of four or five inches. 
Remarks: Spatulatus is not a common species and has only 
been found in the DuPage Riverand Hickory Creek. It is fre- 
quently confounded with zr/s, but that species is almost always 
marked by interrupted rays while in sfa/u/a/us the rays are 
plain and uninterrupted; it is also larger and heavier than 7r/s, 
and the posterior end is more pointed. It seems to be confined 
to the Desplaines River drainage. 
