PRISODON • 1223 



somewhat shining ; pseudocardinals much split, subradial ; na- 

 cre soiled white. 



Length 52, height 28, diam. 23.5 mm. 



Length 61, height 39, diam. 24 mm. 



xA.mazon River. 

 Unio hrownianus Lea, Tr. Am. Phil. Soc, 1838, p. 108, pi. 



XXTV, fig. 116; Obs., n, 1838, p. 108, pi. XXIV, fig. 116. 

 Margarita (Unio) hroumianus Lea, Syn., 1838, p. 13. 

 Prisodon hroumianus Simpson, Syn., 1900, p. 871. 

 Hyria hroivniana Sowerby, Conch. Icon., XYH, 1869, pl- v, 



fig. 12. 

 Mar gar on (Triquetra) hrowniana Lea, Syn., 1852, p. 17; 



1870, p. 26. 

 Triquetra browniana H. and A. Adams, Gen. Rec. Moll., H, 



1857, p. 508. 



Two specimens, the type and another, from the Lea collec- 

 tion are before me and they differ decidedly in several char- 

 acters. The type is the smaller, very solid, much inflated, and 

 has strong, much split, elongated pseudocardinals. The other 

 is larger, less inflated, thinner and has rather small pseudo- 

 cardinals. Its small size, subrhomboid form, and the almost 

 total absence of wings will distinguish it. 



vSubgenus Hyriana Simpson. 1900. 



Hyriana Simpson, Syn., 1900, p. 872. 



Shell solid, compressed, elliptical, with a slight posterior 

 dorsal wing, and a strong one anteriorly, a well-defined but low 

 posterior ridge ending in a point well above the base, inflated 

 at post-basal region, distinctly sulcate ; beaks low, apparently 

 not rayed ; epidermis shining, light yellowish-brown ; hinge 

 line evenly curved ; pseudocardinals numerous, much split into 

 denticles and crooked ; two laterals in the left valve and one 

 (no doubt) in the right; beak cavities very shallow; dorsal 

 scars numerous, anterior scars deep ; nacre whitish, radially 

 grooved along the pallial line. 



Type, Unio ortonii Lea. 



