The naiades of MISSOURI 183 



Miscellaneous Remarks: — According to Simpson this is a 

 puzzling species that closely resembles 0. ellipsis in outline and 

 its great posterior truncation and post-ventral inflation of the 

 female shell would seem to be characteristic enough to make it a 

 good species. Male shells obtained from the Mississippi River 

 of this State, in the hands of the writer, look more like N . ligamen- 

 tina than anything else, yet its cardinals are more stumpy, is a 

 thicker heavier shell and has a rosy nacre. This State is within 

 its range, as its distribution is from Iowa to Kansas. Surber reports 

 "Sanger" (S. canadense) as its fish host. 



Lampsilis Powellii (Lea). 



("Powell's Shell.") 



Not figured. 



1852 — Unio powellii Lea, Pr. Am. Phil. Soc, V., p. 252; Tr. Am. Phil. 

 Soc, X, p. 270, pi. XIX, fig. 25. 



This species is listed for Missouri through a report by Mr. 

 Frierson from a collection made by F. A. Sampson for the Elk 

 River, McDonald Co., this State. Since no specimens are at hand 

 the writer can give no description except that for the shell quoted 

 from Scammon (1906, pp. 288 and 289). 



shell characters. 



External Structures: — "Shell large, rather thin, elliptical 

 in outline, neither inflated nor compressed. Anterior margin almost 

 circularly rounded; ventral margin gently and evenly bowed; 

 posterior margin rounded, biangulated; dorsal margin rather long 

 and straight. Umboidal ratio in specimens examined, 0.20., 

 umbones rather flattened. Anterior and lateral slopes flattened 

 and rounded; posterior slope very slightly excavated and marked 

 with two radial lines. Epidermis smooth and generally shining, 

 olive-brown. Ligament long and rather thick. 



Internal Structures: — "Pseudo cardinals small, serrate, 

 rather bluntly pointed, single in right valve and double in left. 

 Laterals long, slightly curved, lamellar. Interdentum long, narrow, 

 rounded. Anterior adductor cicatrix well impressed, large, much 

 longer than wide; posterior scars large, very slighly impressed, 

 confluent. Dorsal cicatrices on the lower surface of the interdentum. 

 Pallial line well impressed in its one-half and crenulate. Cavity 



