THE NAIADES OF MISSOURI 435 
or ten papillae terminiating toward the center of ventral border 
with longer finger-like tentacles; glochidium unknown, but is 
identical doubtless with that of its sub-species, Brittsv. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell elliptical, medium in size, 
usually rather thin, evenly rounded before; post-umbonal ridge 
not present; beaks large but low sculptured by six inverted V- 
shaped ridges with their apices pointing toward the tips of the 
beaks and with the posterior ridges extending out as longer and more 
oblique rows at the base of the post-ridge region; epidermis smooth, 
shiny, straw-color with coarse broken rays most pronounced 
posteriorly. 
INTERNAL STRUCTURES:—Cardinals erect, double in left, 
single in right valve; interdentum narrow;, laterals double in 
left, single in right, blade-like; beak cavities rather deep and base- 
like; nacre bluish or whitish with tinge of pink in umbonal cavities. 
Sex Length Height Diameter Locality 
God (=) 40. x 27 -imm—(White R:, Hollister) 
SOAS yee Sa 47 Re, 2ocG ty |) # ‘i ) 
Ors lw eXaerd One Key (23h sma (419)? ie Hd ) 
CH AIG), BE DG Se ats} Sivoo Tee Ue ‘: ) 
MISCELLANEOUS REMARKS:—This species is strictly a S. 
Mo. shell and its sub-species is only found in Central Missouri. 
Some of the South Missouri streams bear brevicula that almost 
approach Sbritts1; however, brevicula is a rather common shell in 
the streams of the south slope of the Ozarks, where it is typical. 
Young shells of this species are very thin, while the adult shells 
become very thick comparatively. Perhaps this species exhibits 
sexual dimorphism more than any of this genus. The female had 
a much broader, shorter more inflated shell than the male; the 
latter being more or less biangulated behind. The slightly long 
incurved post-ventral portion of the female shell is very charac- 
teristic. Gravid females are unknown. Srevicula is the largest, 
most emphatic member of the Micromya group. 
Eurynia (Micromya) brevicula Brittsi (Simpson). 
(‘“‘Britts’ Shell.’’) 
PEO MEV EL. “Fags: oo A, and 1B. 
t900a—Lampsilis brittsi Simpson, Pr. Ac. Nat. Sci. Phila., p. 76, pl. 
Vesis mand 2°) ToGo b>, Usis. Nat. Mus. Pro XX, ps 533: 
