186 THE AMERICAN MIDLAND NATURALIST 
double in right; umbonal cavity moderately deep; nacre pearly 
white, irridescent. 
Sex Length Height Diameter Um. ra. Locality 
@) 87 . xX. Fo. xX 1.47) = o170;) (Miss. R.| Hannibal) 
Co 63 xX 45 x 34, —- 0.230. (Des Moines’ R’,,, Dumas) 
O55 x1 940 x 132) —=\Toomso (little Blue w Countney,) 
Oo 33 xX 22° x -20 = o21o” (Des Moines RR Dumas) 
This last measurement of a young shell shows great inflation 
comparatively (See Figs. 56 C and D.) 
MISCELLANEOUS REMARKS:—This species has a_ general 
distribution over the Ohio — Cumberland system. It is not 
uncommon in the Mississippi and Des Moines Rivers, but has 
only been found in one interior stream, the Little Blue, Kansas 
City, where it was collected by Mr. Bush and donated to the U.S. 
National Museum under No. 134,642. Some have found all 
four gills of aesopus marsupial, but most observations seem to prove 
that only the outer are used as marsupia. The writer has only 
observed outer gills as marsupial—even in case of many indivi- 
duals with sterile gills. He is able to verify Dr. Ortmann’s obser- 
vation of charged marsupia with the “‘lilac hue.’ Its accidental 
host has been found to be ‘‘sauger’’ (S. canadense). 
Genus Pleurobema Rafinesque. 
1820—Pleurobema Rafinesque, Monograph of Bivalve Shells of River 
Ohio, Ann. Gen. Sci. Phys. Brux., p. 313. 
1900b—Pleurobema (Raf.) Simpson, Proc. U. S. Nat. Mus., XXII, 
Pp. 745 (amended). 
(Type, Unio clava Lamarck.) 
ANIMAL CHARACTERS:—Anal opening with short mantle 
connection to supra-anal; inner gills much longer, inner laminae 
free from visceral mass; palpi small very pointed; only outer 
gills marsupial; ovisacs distend but little when gravid; conglutin- 
ates white, narrowly leaf-like or lanceolate, not broken; glochidium 
small, spineless, subovate. 
SHELL CHARACTERS:—Shell subtrapezoidal, subquadrate, 
rounded or elongated, upright, or, when oblique, with beaks 
produced anteriorly; beaks usually rather full and high, sculptured 
obscurely with concentric ridges not extending out on the disk; 
disk without sculpture; epidermis olivaceous, reddish brown or 
even black, rays more or less present in umbonal region; hinge 
