I2l6 PRISODON 



right valve and two in the left, which are sometimes obliquely 

 and faintly vertically striate ; external sculpture much as in 

 Hyria. 



Animal unknown. 



Type. Unio stevensi Lea. 



Hyria stkvensi (Lea). 



Shell subtriangular, subcompressed, moderately solid, ine- 

 quilateral ; beaks low, with numerous, subradial ridges, the 

 inner of which coalesce below, the sculpture being extended 

 over the entire shell and zigzagged in front; posterior ridge 

 double, ending in a wide, feeble biangulation below ; dorsal 

 and basal outlines curved ; anterior end rounded, narrowed, 

 very slightly winged above ; posterior dorsal region elevated 

 into a low wing, which is obliquely subtruncated behind ; epi- 

 dermis greenish-brown ; pseudocardinals radial, somewhat split 

 up, sometimes nearly smooth ; laterals remote, delicate, with 

 faint vertical striae; nacre lurid purplish, thicker in front. 



Length 62, height 44, diam. 21 mm. 



Length 64, height 47, diam. 21 mm. 



Yuruari River, Guiana. 

 Unio stevensi Lea, Pr. Ac. N. Sci. Phila., I, 1871, p. 188; Jl. 



Ac. N. Sci. Phila., VIH, 1874, p. 22, pi. vii, fig. 19; Obs., 



Xin, 1874, p. 26, pi. VII, fig. 19. 

 Hyria stevensi Simpson, Syn., 1900, p. 869. 



This species is smaller than any of the others and differs 

 from them all in having regularly radial pseudocardinals. 



It bears much the same relation to Hyria that Castalina does 

 to Tetraplodon, its teeth being more unionoid than those of the 

 other species. 



Genus PRISODON Schumacher, 181 7. 



Triquetra KlEin, Tent. Methodi Ost., 1753, p. 135. 



Prisodon Schumacher (part), Essai Nouv. Syst., 1817, p. 138. 



Paxyodon Schumacher, Essai Nouv. Syst., 1817, p. 139. 



Shell aviculiform, solid, somewhat inflated, with a well- 

 developed posterior ridge, the area above it being excavated, 



