I390 MONOCONDYI.-TiA 



Margaron (Monocondylcra) costulata Lea, Syn., 1870, p. y2. 

 This seems to differ decidedly from any form I am ac- 

 quainted with. The ovate outline and radial ribs are excellent 

 distinguishing characters. 



MoNOCONDYL.i'A coRRTENTESENSis d'Orbigny. 



Shell irregularly short elliptical, convex, rather solid, inequi- 

 lateral ; bealcs almost compressed, slightly elevated ; dorsal out- 

 line arched, subangulate where it joins the obliquely truncate 

 or subtruncate dorsal slope ; anterior end narrow, rounded ; 

 base line much rounded ; posterior ridge rounded, ending in a 

 point above the base line ; surface irregularly striate; epidermis 

 greenish-olive with two or three feeble, green rays on the pos- 

 terior part ; nacre bluish-white, iridescent ; muscle scars im- 

 pressed ; teeth small, stumpy, single in each valve. 



Length 44, height T)'^, diam. 21 mm. 



Argentina. 

 Monocondylcca corricntescnsis d'ORBic.NY, Guer. Mag., 1835, 



p. 38; Voy. Am. Mer., 1843, P- 615, pi. lxvii, figs. 8-10. — 



Clessin. Conch. Cab. Ano., 1876, p. 246, pi. exxviii, figs. 



3, 4. — Simpson, Syn., iqoo, p. 912. 

 Margarita (Mono condyles a) corrientcsensis Lea, Syn.. 1838. 



p. 28. 

 Unio corricntescnsis Haneey, P>iv. Shells, 1843, P- 212, pi. 



xxTi, fig. 15. — Sowerby, Conch. Icon., XVL 1868, pi. xciii, 



fig. 509. 

 Margaron ( Monocondylcca) corrientcsensis Lea, S\ti., 1852. 



p. 45 ; 1870, p. 73. 



More elliptical than reticulata, not so much inflated, and not 

 having the reticulated surface of that species. 



MONOCONDYE.IJA JASPIDEA (Hupe). 



Shell subtrapezoid. compressed, subequilateral, solid ; beaks 

 moderately full and high ; dorsal outline somewhat- arched, 

 subanguiar where it joins the obliquely truncate posterior end; 

 anterior end narrow, rounded ; base curved ; surface covered 

 with squamose stride giving it the appearance and roughness 

 of a rasp; epidermis a tender green with small angular, jasper- 



