Igo THE AMERICAN MIDLAND NATURALIST 
west of the Mississippi Rivers) is a question worthy of investi- 
gation. This heavy, oblique shell is very easy to identify and 
cannot be very well confused with other Pleurobemae. It has been 
defined as ‘‘an overgrown Pleu. clava.”’ 
~ 
Pleurobema obliquum catillus (Conrad). 
(‘Round Pig-toe,’’ “Pink Pig-toe,’’ ‘“‘Osage Nigger-Head.’’) 
Pl XX Pigs) 0a -Asgand ub: 
1836— Unto catillus Conrad, Monog. III, p. 30, pl. XIII, fig. 2. 
1838—Unzto solidus Lea, Tr. Am. Phil. Soc., VI, Pl. V, fig. 13. 
ANIMAL CHARACTERS. 
NUTRITIVE STRUCTURES:—Brachial opening rather large, 
with many papillae; anal smooth, separated from supra-anal by 
very short mantle connection; gills short and wide, inner the wider, 
its inner laminae free from visceral mass; palpi short, wide pointed; 
color of soft parts mostly dingy white. 
REPRODUCTIVE STRUCTURES:—Outer gills only maruspial, 
when gravid brownish, slightly swollen longi-centrad leaving 
ventral edges tapering somewhat obtusely; conglutinates white, 
formed like seeds of an immature cucumber; glochidium inter- 
mediate for caiillus and obliquum as to general form, but larger 
than either, averaging 0.170 x o.180mm. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell trapezoidal to subovate, 
subglobose, thick ,heavy, medium in size, rounded before, obtusely 
rounded behind; disk smooth no radial furrow; beaks high, full, 
sculptured by obscure concentric lines; epidermis of red-leather 
color, rayed anteriorly. 
INTERNAL STRUCTURES:—Cardinals single in right, roughly 
double in left; laterals single in right, double in left valve; scars 
deeply impressed; beak cavities rather deep; nacre solid pink, 
rarely white. 
Sex Length Height Diameter Locality 
Oo 57.5 X 41 xX 33.0mm (Marais des Cygnes R., Rich Hill) 
OME CONT xX Bay Axes Ge (ras if ‘x ” A sian 
eg Kea AP ee WAZOO! (Osage R., Bagnell) 
Or Bian Wok: obs ee ems ee (pay? ’» Baker) 
This last measurement shows this juvenile to be sub-globular. 
Its shell is unusually thick with very shallow beak and branchial 
