NAIADES OF MISSOURI 79 



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, Figs. 62 A and B. 



1836 — Unio catillus Conrad, Monog. Ill, p. 30, pi. XIII, fig. 2. 

 1838 — Unio solidus Lea, Tr.,Am. Phil. Soc, VI, PI. 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 inher 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 catillus and obliquum as to general form, but larger 

 than either, averaging 0.170 x 0.180mm. 



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 



d^ 57.5 X 41 X 33.0mm (Marais des Cygncs R., Rich Hill) 

 9 59 X 42 X 34.5 " ( " " " " " " ) 



cf 45 X 44 X 30.0 " (Osage R., Bagnell) 

 9 27 X 26 X 21.5 " ( " " Baker) 



This last measurement shows this juvenile to be sub-globular. 

 Its shell is unusually thick with very shallow beak and branchial 



