THE NAIADES OF MISSOURI 347 
anteriorly; palpi small, connected one-half of their length antero- 
dorsad; color of soft parts dirty white except for blackish mantle 
edge at siphonal openings. 
REPRODUCTIVE STRUCTURES:—Marsupia rather low on post- 
ventral portion of outer gills; when gravid the numerous distinct 
ovisacs extend below the original edge of gill; glochidia smallest 
on record, measuring 0.600 x 0.063 min., semicircular, spineless, 
hinge line short, very slightly undulated; conglutinates white, loosely 
connected when glochidia mature. 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:— Shell very small, compressed, 
rounded before, pointed high behind; post-umbonal ridge rather 
prominent; disk smooth; beaks rather full sculptured by five 
upward angled bars extending out as finer concentric lines in later 
bars; epidermis green or olivaceous, painted with radiating green 
rays of zigzag or arrow-marks; female shell inflated post-ventrad. 
INTERNAL STRUCTURES:—Cardinals double in left, single in 
right valve, compressed, high and ragged; interdentum narrow; 
laterals single in right, faintly double in left; nacre pearl blue 
or white. rarely pink. 
Sex Length Height Diameter Locality 
Cis 4k sO x 22mm (Grand R., Gallatin) 
OP Grok (22) 0. Simin (Platte R., Dixon Falls) 
OR sche 2 OU aXe e ORA TT TIT ((heges ” Agency Ford) 
He LAG X25 ook 9.5mm (Grand R., Darlington) 
OVO TATE. x TORK AS5 sr tTd be (oe ”’ Chillicothe) 
The last two are among the smallest juveniles in the writer’s 
collection. The smaJlest meets with the following description :— 
Post-umbonal ridge sharply angled; beaks prominent, well up 
toward middle of dorsal line, sculptured by five early bars bowed 
upward in the center, the latter ones being rather fine, concentric: 
epidermis green with costa-like paintings on the post-dorsal 
ridge; disk with two rows of zigzag paintings parallel to the growth 
lines. Four juveniles of this species were found in the Osage 
clinging to one byssal thread, but unfortunately they were lost. 
MISCELLANEOUS REMARKS:— Typical donactformis may be 
easily distinguished from Amyg. truncata in possessing a smaller 
shell, more painted, thinner, less inflated, more dorsally ridged, 
and is more of an inhabitant of quiet water with mud-sand bottom. 
Donaciformis is more supplanted in Central Missouri by truncata 
