192 THE AMERICAN MIDLAND NATURALIST 
ANIMAL CHARACTERS. 
NUTRITIVE CHARACTERS:—Branchial opening with two ranks 
of papillae; anal crowded with fine, short papillae; supra-anal 
moderately separated from anal; inner gills slightly wider than 
outer, inner laminae free from visceral mass; palpi thick and long; 
connected antero-dorsad over half of length; color of soft parts 
dingy white, for most part. 
REPRODUCTIVE STRUCTURES:—Only outer gills marsupial: 
when gravid marsupia cream colored, somewhat padiform; conglu- 
tinates white, leaf-like, solid; glochidium medium, subovate 
spineless, measures 0.150 X 0.155mm. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell roundly elliptical, thick and 
solid anteriorly, moderately thin and compressed for posterior 
half; post umbonal ridge flattened; beaks not prominent, nor 
protruding anteriorly, sculptured by concentric corrugations; 
epidermis reddish brown, rayed anteriorly. 
INTERNAL STRUCTURES:—Cardinals double in both valves. 
laterals single in right, double in left valve; interdentum wide and 
thin; beak cavities shallow; nacre rose pink. 
Sex Length Height Diameter Locality 
Ce i7 Ay bee O20 bur oa! (Osage R., Warsaw) 
OF) (OAR ERC NAN ie eT (hee e y ) 
Ge AO! pee 137 aexue Co ” . Baker) 
MISCELLANEOUS REMARKS:—No juveniles of this form are 
at hand for description. The writer met with this variety in the 
Osage. It can be easily distinguished from the more typical cocci- 
neum of South Missouri in not possessing such compression for 
the posterior half of shell, in being more elliptical and heavier. 
The general outline is that of O. ellipsis. It can be separated from 
Pleu. obliquum catillus by this elliptical shape, but its greater 
compression (especially posteriorly) and by its thinner, less solid 
shell. The writer made an interesting discovery while examining 
a gravid individual of this form afield with a (X12) lens to find its 
late embryos in rapid rotary motion around one axis. This phe- 
nomenon had been observed by the author in Lastena ohiensis 
A breeding record of this form shows it to be a summer breeder 
3 
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