1274 inpi.onoN- 



Castalia pazi Hidalgo. Jl. cle Conch., X\I, 1868, p. 353, pi. 



XIII, fig. 6. 

 Diplodon pad Simpson, Syn., 1900, p. 884. 



Not so distinctly biangulate behind as D. Iiylccits, the sculp- 

 ture nodulous and inclined to break up below into a zigzag 

 pattern. I have no doubt but what this is a Diplodon. 



Diplodon HVL-iius (d'Orbigny). 



Shell somewhat rhomboid, solid, convex, inequilateral, with 

 a nearly straight dorsal line ; the post-dorsal slope having a 

 straight oblique truncation ; posterior ridge distinctly double 

 below, ending behind a little above the base in a decided bian- 

 gulation ; post-basal region a trifle swollen ; surface covered 

 for about half way down from the beaks with strong, curved, 

 radial ridges, one or two of the central pairs coalescing; the 

 outer portion of the shell appears to have sulcate, concentric 

 sculpture ; epidermis greenish-brown ; teeth apparently rather 

 strong ; nacre bluish-white. 



Length 44. height 24, diam. 17 mm. 



Bolivia ; Paraguay River. 

 Unio hylcra d'ORBiONY, Guer. Mag., 1835, p. 36; Voy. Am. 



Men, 1843, P- ^'^7> P^- Lxix, figs. 8, 9. 

 Margnron (Unio) hylcrus LtvA, Syn., 1852, p. 21 ; 1870, p. 31. 

 Unio kyl'Cius Sowerby, Conch. Icon., XVI, 1868, pi. xciii, fig. 



506. 

 Diplodon hylceaus Simpson, Syn., 1900, p. 884. 

 Unio guaraniana d'ORBiGNY, Guer. Mag., 1835, p. 37; Voy. 



Am. Men, 1843, P- ^^' P^- i-xix, figs. 10-12. 



Unio guaraniana d'Orbigny seems to me to be only a young 

 hylcens. The post-dorsal point is a little more elevated than 

 that of the last named. 



Group of Diplodon parallelipipcdon. 



Shell elongated, subtrapezoidal, inflated, obliquely truncate 

 behind, posterior ridge strong ; beaks rather low, sculpture of 

 nearly or quite strictly radial bars with concave spaces be- 

 tween ; epidermis dark; pseudocardinals compressed in the 



