200 THE AMERICAN MIDLAND NATURALIST 
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 
ropWen col sual s.e) nclo) ero apherdh toahacl (Miss. R., Hannibal) 
CeO was Oak aaa GNise ” ” ) 
PUTTS ky ON eR) Az (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. 
