246 THE AMERICAN MIDLAND NATURALIST 
Fic. 5. Mature closed glochidium of U. tetralasma. (x87) 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell subtrapezoid; post-dorsal 
ridge rather high, usually obtusely pointed behind, evenly rounded 
in front; disk smooth; beaks low, drawn well back from anterior 
end, sculptured with many coarse, concentric, regular ridges 
curved up very abruptly behind at base of post-umbonal ridge 
where the ridges are drawn closely together; epidermis light 
yellow back ground with alternated brown bands running parallel 
with the growth lines, or nearly all colored in brown horn with 
polished appearance, almost rayless, sometimes faintly rayed 
in green on post-umbonal ridge. 
INTERNAL STRUCTURES:—Cardinals compressed into rather 
blade-like processes; laterals delicate but rather prominent; 
interdentum long, thin; umbonal cavities rather shallow; scars 
well impressed, nacre light bluish to grayish. 
Sex Length Height Diameter Um. ra. Locality 
So 95 X 45 X 31 mm _ 0.260 (Batterton Pond, Columbia) 
Oo. $80 x tragie xt ges?) vemGsy -Chost reris pAgaty) 
OTT Gia XA Die XP OA shag 0.260 (Mill Creek, Courtney) 
OP aot EX sy TO eek, wa) Re 0.270 (Lost Creek, Amity) 
MISCELLANEOUS REMARKS:—U. tetralasma is peculiar ecolo- 
gically, as well as morphologically, in that it can become more 
quickly established in artificial ponds and lakes than any species. 
It is naturally lacustrine, but for some unknown cause it is not 
found in any of our lakes prefering small ponds or quiet creeks 
where it is found accompanying Anodonta Danielsii, or Eurynia 
subrostrata. From the fact that the writer has not found any 
individuals of this species in North Missouri without marsupial 
characters he is led to believe that it is locally hermaphroditic at 
