268 NEW UNIONID^, MELANIDiE, ETC., 



Shell smootb, elliptical, somewliat inflated, very inequilateral, rounded before and 

 behind ; valves rather thick, thicker before ; beaks slightly prominent ; epidermis 

 greenish brown, without rays ; cardinal teeth somewhat thick, compressed and double 

 in both valves ; lateral teeth long, lamellar and somewhat curved ; nacre silver white 

 and very iindescent. 



Proc. Acad. Nat. Sci. 1866, p. 38. 



Hah. — South America, Don Patricio M. Paz. 



My cabinet and cabinet of Don P. M. Paz. 

 Diam. 1, Length 1-4, Breadth 2-4 inches. 



Shell smooth, elliptical, somewhat inflated, veiy inequilateral, rounded before and 

 behind; substance of the shell rather thick, thicker before; beaks slightly promi- 

 nent; ligament rather long and light brown; epidermis greenish brown, without rays, 

 with distant marks of growth; umbonial slope rounded and inflated; posterior slope 

 narrow, elliptical, and very slightly carinate, with indistinct impressed lines in each 

 valve; cardinal teeth somewhat thick, compressed and double in both valves; lateral 

 teeth long, lamellar and somewhat curved ; anterior cicatrices confluent, rather small 

 and deeply impressed ; posterior cicatrices confluent, large and slightly impressed ; 

 dorsal cicatrices placed in a row across the centre of the cavity of the beaks; cavity 

 of the shell rather shallow and wide ; cavity of the beaks shallow and rounded ; 

 nacre silver white and very iridescent. 



Soft parts. — Branchial uterus occupies nearly the whole width of the inner branchiae, 

 the ovisacks being long, closely approximate and filled with nearly mature embryonic 

 shells. Ova were found still in the ovaries. Brancldoi rather large, outer ones much 

 rounded, inner ones larger and nearly straight at the basal edge, united the whole 

 length of the abdominal sack. Pa/ju" rather small, rather thick, obliquely rounded 

 below and united a short distance on the posterior edges. Mantle thin, much thick- 

 ened at the posterior edges. Branchial openinrj rather small, dark brown, with regu- 

 lar, rather small papillte. Anal opening] very small, without papillae and entirely 

 disunited from the branchial opening below. No super-anal opening. Color of the 

 mass dirt}' white. 



Embryonic shell subtriangular, light brown ; dorsal line rather long and straight; 

 side margins slightly curved outward, forming an obtuse angle at the base; basal 

 margin obtusely angular and furnished with hooks ; granulate over the whole sur- 

 face. PI. 34, fig. 82. 



Remarlcs. — Four specimens of this species were brought by Don P. M. Paz, two of 

 them, a male and female, in alcohol, and from which the above descriptions were 

 made. In connection with the description of the embryonic shell it may be men- 

 tioned that it is, I believe, the first time this has been done in regard to South Ameri- 

 c.ui species. In outline y?/-?nHs is closely allied to caliginosus (nobis), but it is a heavier 



