30 NAIADES OF MISSOURI 



times shows a tendency to bianguation posteriorly; beaks incurved, 

 projecting forward, sculptured with concentric lines but no sculp- 

 turing carried out ort the disk; rest lines of growth concentric 

 and furrowed; epidermis satiny horn color. 



Internal Structures: — Cardinals large and massive; lat- 

 erals long and heavy; interdentum broad; beak cavities deeply 

 creviced; scars deep; nacre pure white, stippled, irridescent. 



Sex Length Height Diameter Um. ra. Locality, 



cf 105 X 75 X 60mm 0.090 (Miss. R., La Grange, Mo.) 



9 95 X 65 X 57mm o.ioo ( " " " " " ) 



cf 56 X 46 X 32mm 0.120 ( " " Hannibal, Mo.) 



cf 20 X 18 X 8mm 0.115 ( " " " " ) 



The shell of the juvenile of this last measurment is so round, 

 and with beaks so drawn up to center of dorsal line that it resembles 

 that of a Sphaermm. Umbonal sculpturing not very distinct 

 even in this juvenile. It can always be identified in the early 

 stages of its juvenility by white spots located post-dorsad on its 

 shell. 



Miscellaneous Remarks :^ — The writer, after some years of 

 extensive collecting from the largest streams of the state, has 

 failed to find ehena in the interior streams, neither have any Mis- 

 souri collectors, nor old "clammers" reported it. This shell is 

 known not only for its greatest commercial value and for its rarity 

 in general geographical distribution but also for its great abundance 

 locally. Of course its only home, known so far, is the Mississippi 

 north of the Missouri River. It is not known why this species 

 does not occur for this state in those Ozark rivers that bear it in 

 great abundance in Arkansas not far from the Missouri-Arkansa'^ 

 line. Sometimes a black shelled Pleurobema pyramidatum or 

 Fusconaia undata trigonoides may be taken for the "Nigger Head" 

 (F. ehena), but, from the characteristic cornucopia-form of shell, 

 together with its deep brown satiny epidermis and regular, con- 

 centric furrowed rest lines of growth, it should be easily identified. 

 Frierson reports ehena as a rare shell in Louisiana and Isely (19 14, 

 pp. 1-4) does not report it at all for the Arkansas and Red River 

 drainages of eastern Oklahoma. Perhaps Call's account of it (1895, 

 p. 16) as a common shell, not only for Arkansas, but for all the 

 larger rivers west of the Mississippi, is more conjectured than 

 real. Its breeding season has been found by Wilson and Clark 

 (1914, p. 42) to extend from May to the middle of July. Surber 



