2-- 



THE NAIADES OF MISSOURI 89 



with numerous short papillae; anal opening with small, but very 

 distinct, papillae; supra-anal briefly but well connected with 

 anal by mantle edges; gills large, broad, pointed both anteriorly 

 and posteriorly, inner broader, only slightly longer, than outer, 

 inner laminae free from visceral mass; palpi moderately large, 

 connected antero-dorsad, edges curved; color of soft parts dirty 

 white with mantle edges at siphonal openings blackish and gills 

 brownish. 



Reproductive Structures: — Only outer gills maruspial, 

 when gravid marsupia moderately swollen; conglutinates rather 

 well developed, leaf like, white; glochidia small, suboval, spineless, 

 measures 0.130 x 0.150mm. 



SHELL characters. 



External Structures: — Shell solid, heavy, subinflated, tri- 

 angular in outline, post-umbonal ridge prominent; beaks also 

 prominent sculptured by a few rather obscure ridges subparallel 

 to growth lines and swollen at the base of post-ridge ; disk more or 

 less smooth; epidermis reddish brown to black often faintly rayed. 



Internal Structures: — Cardinals double in both valves, 

 heavy; laterals heavy; interdentum long; beak cavities large 

 not very deep; nacre (for Mo. nigra) only a deep purple — not 

 variable. 



Sex Length Height Diameter Locality 



cf 126 X 80 X 51mm (Miss. R., Hannibal) 



c?' 91 X 61 X 37 " ( " " " ) 



9 115 X 66 X 47 " (Mermamec R., Meramec Highlands) 



Miscellaneous Remarks:— No juveniles, nor young shells 

 found. This is a rare species for the interior of the state being only 

 in the Meramec, outside of the Mississippi River. In the latter, 

 where it is not to say an uncommon shell, it is different from the one 

 found in the Ohio by a variation in nacre-color. The shell may 

 show some variation in size and form in the same river, as Wilson 

 and Clark (1914, p. 42) observed in the Cumberland where it is 

 short and chunky in the headwaters but is heavier and more 

 elongate in the lower stretches. Although nigra is essentially a 

 big river species, yet it is not found in the Osage — the largest 

 Missouri River tributary. Its occurence in the Meramec carries 

 it farther west than recorded before. Its breeding record, although 

 incompletely kept by the writer, shows it to be a tachytictic Unio. 



