THE NAIADES OF MISSOURI 



95 



l"'iG. 5. Mature closed glochidium of JJ. tetralasma. (xSy) 

 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 



cf 95 X 45 X 31 mm 0.260 (Batterton Pond, Columbia) 



9 88 X 44 X 32 " 0.265 (Lost Creek, Amity) 



9 76 X 42 X 24.5 " 0.260 (Mill Creek, Courtney) 



9 23 X 12 X 8 " 0.270 (Lost Creek, Amity) 



nbia) ^ ' 



Miscellaneous Remarks:— L". tetralasma is peculiar ecolo-^' , W ' 



gically, as well as morphologically, in that it can become more nCto Jj-* .. 

 quickly established in artificial ponds and lakes than any species. U^*V''^il \ 

 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 

 suhrostrata. From the fact that the writer has not found any 

 individuals of this species in North Mis.souri without marsupial 

 characters he is led to believe that it is locally hermaphroditic at 



