THE NAIADES OP MISSOURI I49 



more rounded out branchial cavity and somewhat coarser cardinals 

 these structures are identical with those of donacijormis. 

 Sex Length Height Diameter Locality 



c?' 50 X 36 X 25 mm (Osage R., Linn Creek) 



9 43 X 34 X 23 mm (Gasconade R., Gascondy) 



9 38 X 30 X 17 mm (Osage R., Schell City) 



cf 37 X 27 X 16.5mm ( " " Proctor) 



Miscellaneous Remarks: — As pointed out under the des- 

 cription of donacijormis, truncata is as distinct when typical as 

 its only con-generic ally — especially in its broadly truncated shell 

 post-dorsad. Neither is truncata so sexually dimorphic; however, 

 the male shell is unusually more elongated and pointed posteriorly 

 and has less inflation. Both sexes have their shells rather expanded 

 post-ventrad. This species is rarely found in South Missouri, 

 (the writer not having made personal collections of it there at all) 

 although it is really more of a southern species than donacijormis. 

 It is not to say very typical in the Osage or Gasconade basins 

 where there are many intergrades for it and its ally, but reaches 

 its greatest perfection in the Mississippi. The breeding habits 

 of this species is the same as that of donacijormis as far as records 

 show. 



Genus Plagiola Rafinesque. 



1819 — Plagiola Rafinesque, J. de Phys. Chim. Hist. Nat., p. 426; 

 1852, Agassiz, Arch, Fur Nat., p. 48, (redefined); 1900b, Simpson, Pr. 

 U. S. Nat. Mus., XXII, p. 603. 



(Type, Unio securis Lea.) 



Animal Characters: — Anal opening smooth, connected to 

 supra-anal by close mantle attachment; inner laminae of inner 

 gills free or partly connected to visceral mass; gills brownish- — 

 all other soft parts tannish; marsupium rather reniform consisting 

 of 40-50 well defined ovisacs; conglutinates lanceolate, not very 

 solid; glochidium spatulate, very much higher than long, spineless, 

 very large. 



Shell Characters: — Shell sub-triangular, solid, not greatly 

 inflated, with square cornered post-umbonal ridge and flat post- 

 dorsad; disk smooth; beaks pointed, rather high, sculptured 

 with faint double-looped ridges; epidermis yellow with broken 

 rays; cardinals low and jagged; laterals rather stout, straight or 

 slightly curved; nacre white. 



Miscellaneous Remarks: — The chief characteristic of this 



