THE NAIADES OF MISSOURI 9 1 



The last two measurements are those of the most adolescent 

 shells at hand but are not young enough to present much addi- 

 tional information than can be obtained from the mature shell. 



Miscellaneous Remarks: — This is one of the species of very 

 wide geographic distribution in United States but is limited in 

 this state for the interior to those streams south of the Missouri 

 River where it is met with in a multitude of forms — especially 

 of the small thin and compressed varieties found in the Ozarkan 

 streams of South Missouri. In the Osage 'it is found most typically, 

 aside from the Mississippi which produces the best types of all 

 Naiad species. The depauperization of this species in size and form 

 in the mountain streams is evidently due to a swifter current and 

 hence these dwarfed forms may be merely a local ecologic race 

 that may not deserve special names; however, two forms in this 

 state occur often enough to require some little attention. 

 Dilatata (Raf.) {=gibbosus (Bar.)) is more confused with Ellip- 

 saria clintonensis (Simpson) than with any other shell. There is a 

 difference, however, in the latter possessing capillary rays, and 

 a broader interdentum, but the best distinction is in the marsupial 

 structures of the latter that presents a folded appearance, when 

 gravid. Simpson did not know the difference from shell structures 

 until he found a gravid clintonensis. While this species is both 

 lacustrine and fluviatile, yet it is never found in any lake or pond 

 in this state. The typical dilatata with white nacre {E. arctior) is 

 simply put down in the synonomy. A form like Conrad's U. arcus 

 is sometimes found in the Osage but aside from its shortened 

 dwarfed form of shell it does not even possess enough differentia- 

 ting characters to give it a varietal place. The writer has found 

 it to be gravid only from June to August; hence tachytictic. 



Elliptio dilatata subgibbosa (Lea). 

 ("lyittle Lady Finger," "Little Spike.") 

 PL XXL, Figs. 68 A—D 



1857 — Unio subgihbostis Lea, Pr. Ac. N. Sci. Phila., IX, p. 169; 1858, 



JL Ac. N. Sci. Phila., IV, p. 53, pi. VI, fig. 36. 

 1868 — Unio lazarus Sowerby, Conch, Icon., XVI, pi. LXVIII, fig. 348. 



Animal Characters: — Absolutely identical with those of the 

 parent. No real difference in glochidial characters even. 



Shell Characters: — Moderately small, short, rather in- 

 flated, somewhat heavy through the post-ridge, more elliptical, 



