68 NAIADES OF MISSOURI 



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 PletJtobasus cooperianus as far as shell 

 structure is concerned. 



Rotundaria tuberculata (Rafinesque). 



("Purple Warty Back," "Red Nigger Head.")) 



PI. XIX, Figs. 54 A and B; PI. I, Figs. 1—4. 



1820 — Obliquaria {Rotundaria) tuberculata Rafinesque, Ann. Gen. Sc. 

 Brux., v., p. 103. 



1898- — Quadrula verrucosa Baker, Moll, Chicago, Pt. I, p. 85, pi. XXIII. 

 1900b — Quadrula tuberculata Simpson, Proc. U. S. Nat. Mus., XXII. 



P- 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, 



