THE NAIADES OF MISSOURI I47 



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 o.o6j 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 

 22mm 

 15mm 

 14mm 



9.5mm 



4.5mm 



The last two are among the smallest juveniles in the writer's 

 collection. The smallest 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 donacijormis 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. 

 Donacijormis is more supplanted in Central Missouri by truncata 



