NAIADES OF MISSOURI 77 



nated post-ventrad with radial furrow in front of flattened post- 

 umbonal ridge; beaks swollen, protruding anteriorly, sculptured 

 by concentric ridges; epidermis reddish brown to black with rays 

 originating in umbonal region; cardinals heavy, double in left 

 single in right valve; laterals double in left, more or less double 

 in right valve; nacre white. 



Miscellaneous Remarks: — The writer has been unable to 

 find Pleu. ohliquwm in typical form anywhere in the state and 

 claim is only made for it through its various forms and through 

 Simpson's report, that it is found in the Mississippi above the 

 mouth of the Missouri River. vSurely there is much need of re- 

 search chiefly with regard to the geographic facts relating to the 

 distribution of this species. Most students of geographic distribu- 

 tion concur in the belief that no true ohliquum is found west of the 

 Mississippi and that it is rarely seen north of the Ohio. Its metrop- 

 olis is that richest of all centers of mussel faunae, the Cumberland 

 River, where Wilson and Clark report it as the most abundant of 

 all the numerous species found there. Briefly stated, ohliquum 

 occurs in this state, but with its characteristic radial furrow obliterated 

 and its intergrading forms are very numerous running into each 

 other in every puzzling way. 



Pleurobema obliquum plenum (Lea). 

 Not figured. 



1840 — Unio plenus Lea, Tr. Am. Phil. Soc, I, p. 286; Tr. Am. Phil. 



Soc, VIII, 1843, p. 211, pi. XIV, fig. 26. • 

 1900b — Quadrula plena Simpson, Proc. U. S. Nat. Mus., p. 790. 



The writer has not found this species in this state and no 

 description appears here, since neither soft parts nor shells have 

 been seen. However, plenum is listed as a variety for Missouri 

 through the kind report of Mr. Walker that he has it from the 

 James River, near Springfield, and that it bears the same relation 

 to ohliquum as does also a variety of coccineum found in the same 

 locality. 



Pleurobema obliquum pyramidatum (Lea). 

 ("Pig-toe," "Pyramid Pig-toe.") 

 PI. XX, Figs. 58 A and B. 

 1834 — Unio pryramidatus Lea, Tr. Am. Phil. Soc, lY, p. 109, pi. XVI, 



fig- 39- 

 1900b — Quadrula pyramidata Simpson, Proc. U. S. Nat. Mus., XXII, 

 p. 790. 



