150 THE AMERICAN MIDLAND NATURALIST 
SHELL CHARACTERS:——Shell rotund, disk sculptured for two- 
thirds of posterior part with irregularly placed tubercles, slopes of 
post-dorsal ridge regularly costated, some costae somewhat parallel 
to umbonal ridge behind; beaks well placed anteriorly, sculptured 
profusely with concentrically zigzag lines across two obliquely 
posterior ridges being coarser in the valley between; nacre purple; 
cardinals heavy, double in left, more inclined to be single in right; 
beak cavities deep antero-postero, narrow diametrically, wide 
vertically. 
MISCELLANEOUS REMARKS:—This genus is unique for its 
absence of supra-anal opening, for its limitation of marsupia to 
the outer gills and for its peculiar shell structure in the presence 
of a well developed sulcus at the post-dorsal part and in its remar- 
able beak sculpture. The two species of this Genus are most typically 
represented in the Mississippi of this State, but are found to be 
more intermediate in form for Central and South Missouri. In 
the Gasconade and Osage basins these forms grade into those 
that may be referred to Plethobasus cooperianus as far as shell 
structure is concerned. 
Rotundaria tuberculata (Rafinesque). 
(‘Purple Warty Back,’ ‘““Red Nigger Head.’’)) 
PI. XIX, Bags. 54 Axand By PLT thas. Sie 
1820—Obliquaria (Rotundaria) tuberculata Rafinesque, Ann. Gen. Sc. 
BraxctiVer tps 1O3r 
1898—Quadrula verrucosa Baker, Moll, Chicago, Pt. I, p. 85, pl. XXIII. 
1900b—Quadrula tuberculata Simpson, Proc, U. S. Nat. Mus., XXII. 
Pp. 795. 
1912b—Rotundaria tuberculata (Raf.) Ortmann, An. Car. Mus., VIII, 
pp. 258-259. 
ANIMAL CHARACTERS. 
NuTRITIVE STRUCTURES:—Branchial opening very large with 
few short simple papillae; anal as large as branchial with still 
shorter papillae; rectum large, visible, anus slightly tentacled; 
gills wide, short, tilted at abrupt angle, inner much wider, inner 
laminae free from visceral mass; palpi connected over half of 
their length antero-dorsad, pointed postero-ventrad; soft parts 
dingy wihite, for most part, gills dark brown black posteriorly, 
foot dark tan. 
REPRODUCTIVE STRUCTURES:—Only outer gills marsupial, 
