i ip 4 THE AMERICAN MIDLAND NATURALIST 
connected to anal, mantle edge here blocked in black at regular 
intervals; inner gills much wider and longer than outer, inner 
laminae connected about one-half way; palpi united almost to 
their tips antero-dorsad; color of foot, palpi and adductors orange, 
variable with age. 
REPRODUCTIVE STRUCTURES:—Marsupia occupying outer gills 
with secondary water tubes, ventral edges distended when gravid, 
ovisacs occupied by several other smaller sacs arranged crosswise 
facing the outer and inner laminae in which small, solid white 
cords (placentulae), containing the ova or two to ten larvae, are 
situated; glochidia large, spined, spadiform, hinge line straight, 
length greater than height, (0.35x0.285 min). 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Elongate-ovate, moderately solid, 
inflated, post-umbonal ridge usually rounded; female shell more 
obtusely (or biangulated) posteriorly than male; umbones rather 
full sculptured by two or three very coarse, wavy concentric 
ridges abruptly bent up behind; disk not sculptured; epidermis 
usually a glossy brown, sometimes marked by bright green rays, 
especially in young shells. 
INTERNAL STRUCTURES:—Cardinals almost obliterated, more 
pronounced in left valve, where it is rounded and placed just 
under the beaks; laterals and interdentum lacking; umbonal 
cavities deeper in female shell; nacre variable from solid salmon 
or white, to cream or pearl-blue color. 
Sex Length Width Diameter Um. ra. Locality 
9 95 x 55 x 38—mm—o.360—(Marais des Cygnes R., 
Rich Hill, Mo.,) 
oO 77.5 X 45.5 X 27 —-mm—o.430 (Osage R., Linn Cr.,) 
O34. X 20° Xe 14 —=min—-0.330)) (10o2eR:) Wyeth, low 
St AT x2 xX 13.5—mm—o.335 (Grand R., Darlington.) 
The last two measurements are those of the only shells of eden- 
tulus found in the interior north of the Missouri River and these are 
yuveniles. Unlike most adolescent shells of this species, both shells 
are uni-colored except for a single brownish band. parallel with 
the growth lines near the ventral margin on a back-ground of 
yellowish-green; hinge teeth and beak sculpture typically stro- 
phitus; nacre of both about the same; shell thin and transluscent. 
MISCELLANEOUS REMARKS:—S. edentulus is rather easily 
recognized even through casual observation by its somewhat 
