PARRKYSIA 1 1 05 



Elliptic rhomboid. 



Sculpture extending only part way over the disk. 



Group of P. rugosa. 

 Nearly smooth or very finely sculptured. 



Group of P. corbis. 

 Strongly sculptured. Group of P. leopoldvillensis. 



Triangular. 



Posterior ridge high ; surface sculptured. 



Group of F. nyassccnsis. 

 Beaks high ; posterior ridge low. Group of P. corrugata. 

 Shell subquadrate, concentrically sculptured. 



Group of P. niolleiiri. 

 Shell rhomboid, very small, solid, inflated. 



Group of P. fabagina. 



Subgenus Parreysia s. s. 

 Characters the same as the genus. 



Group of Parreysia corrugata. 



Shell having the middle of the basal region swollen, beak 

 sculpture strong, the central ridges generally united at their 

 lower points to form chevron-shaped markings. 



Parreysia coRRUG.vrA (Miiller). 



Shell variable in outline, generally somewhat elliptical, sub- 

 inflated or inflated, rather solid, more or less inequilateral ; 

 beaks full, quite elevated, sculptured with radial ridges, which 

 are more or less zigzagged, and sometimes extend well out over 

 the disk ; anterior end more or less narrowed, rounded ; base 

 rounded, usually fullest behind the middle ; dorsal outline ar- 

 cuate ; outline of dorsal slope obliquely subtruncate, some- 

 times obliquely rounded, with an angle where it meets the 

 dorsal outline ; posterior ridge usually full and subangulate, 

 surface with irregular growth lines, which are developed into 

 ridges in front ; the upper anterior region and the dorsal slope 

 often have irregular, radial plications ; epidermis generally 

 smooth and shining, yellowish, tawny, straw-colored or brown- 



