THE NAIADES OF MISSOURI 349 
more rounded out branchial cavity and somewhat coarser cardinals 
these structures are identical with those of donactformis. 
Sex Length Height Diameter Locality 
ote YO Sa) AG) Bs AS mioteel (Osage R., Linn Creek) 
OF A GWEESon To AGM x me 2.36) TTT: (Gasconade R., Gascondy) 
CPP RST Se coy mc TGA Mriahaal (Osage R., Schell City) 
CHR sc WIT woos 5 alee Sioob an (oe ’”’ Proctor) 
MISCELLANEOUS REMARKS:—As pointed out under the des- 
cription of donaciformtis, 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 donaciformts. 
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 donaciformis as far as records 
show. 
Genus Plagiola Rafinesque. 
1819—FPlagiola Rafinesque, J. de Phys. Chim. Hist. Nat., p. 426; 
1852, Agassiz, Arch, Fur Nat., p. 48, (redefined); t1go00b, 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 
