TrCTRAPLODON I 20 1 



in a blunt point near the base of the shell ; above the posterior 

 ridge on the decidedly truncate posterior slope there is a 

 broad, radial furrow, which ends in a slight sinus; dorsal slope 

 subangular behind the ligament; pseudocardinals solid, sub- 

 compressed, rough but not split ; laterals somewhat vertically 

 striate; beak cavities deep, round; anterior scafs deep; nacre 

 white, iridescent behind. 



Length 65, height 49, diam. 41 mm. 



Guiana; Amazon? 

 Castalia ambigua Sowerp.v, Conch. Man.. 1839, fig. 140. 

 Castalia retiisa Hupe, Mol. Nouv., Ill, 1857. p. 75, pi. xiv, fig. 



2. — SowERBY, Conch. Icon., XVII, 1869, pi. i, fig. 2. 

 Tetraplodon rctusus Simpson, Syn., 1900, p. 864. 



Close to T. aiiibiguits and it may be only a variety of that. 

 It dififers in having only a few, wide radial ridges on the 

 umbones, which extend but a short distance over the disk, in 

 being strongly and irregularly concentrically sculptured, and 

 in having more compact, less split-up pseudocardinals. 



Tetraplodon ouadratu.s (Sowerby). 



Shell solid, subquadrate or subrhomboid, inequilateral ; beaks 

 very full and high, turned forward, with a few strong radial 

 ribs, which extend about one-third of the distance from the 

 beaks to the base, the rest of the shell nearly smooth; posterior 

 ridge much elevated, decidedly rounded, apparently bordered 

 by sulci at each side, ending in a point at the base of the shell ; 

 epidermis straw-colored : teeth strong ; anterior scars deep, 

 small; nacre apparently ]Mnkish. 



Length 26, height 22 mm. 



Guiana. 

 Castalia qiiadrata Sowerby, Conch. Icon., XVII, 1867, pi. ir, 



figs. 7, a, b. 

 Tetraplodon quadratiis Simpson, Syn., 1900, p. 864. 



This may be only the young of a form, which becomes much 

 larger when mature, but the shell is described as solid. Its 

 smooth, straw-colored surface and the rounded cord-like pos- 

 terior ridge should distinguish it from any others. 



