172 THE NAIADES OF MISSOURI 



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, Brittsi. 



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 



27 mm — (White R., Hollisterj 



22.5mm — ( " " " ) 



23 mm — ( " " " ) 



18 mm — ( " " " ) 



Miscellaneous Remarks: — This species is strictly a S. 

 Mo. shell and its sub-species is only found in Central Missouri. 

 vSome of the South Missouri streams bear hrevicula that almost 

 approach Brittsi; however, brevicula is a rather common shell in 

 the streams of the south 3lope 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. Brevicula is the largest, 

 most emphatic member of the Micromya group. 



Eurynia (Micromya) brevicula Brittsi (Simpson). 



("Britts' Shell.") 



PI. XXVII; F.igs. 99 A and B. 



1900a — Lampsilis brittsi Simpson, Pr. Ac. Nat. Sci. Phila., p. 76, pL 

 v., fig. I and 2; lyoo b, U. S. Nat. Mus. Pr., XXII, p. 533- 



