ortmann: south American naiades. 577 



44. MoNOCONDYL^A PARAGUAYANA D'Orbisny (1835). 



Monocondylwa paraguayana D'Orbigny, 1843, p. 612, PI. 70, figs. 5-7; Simpson, 



1900, p. 911 (in part); 1914, p. 1387. 

 Unio paraguayana Sotvtdrby, XVI, 1866, PI. 52, fig. 273. 



Type-locality. — Rio Parand, Itaty, near Corrientes, Argentina. 



Other Locality. — Ptio Batel, Province Corrientes, Argentina. 



Neiv Locality. — Rio Uruguay (in mud), Uruguayana, Rio Grande do Sul, 

 Brazil (J. D. Hascman coll.-, February 5, 1909). One female with soft parts. 



Distribution. — Known from Rio Parana and its tributary, the Rio Batel in 

 Argentina, and from the Rio Uruguaj'. 



Characters of Shell. — Shell moderately thick, and of medium size (maximum 

 length 59 mm.). Outline briefly subtrapezoidal, rather high (height 77 to 81 pr. ct. 

 of length), distinctly oblique, but approaching an angularly suborbicular shape. 

 Valves in front gaping very little. Dorsal margin behind the beaks slightly convex, 

 much lower in front of the beaks, where it is distinctly concave, and passes into the 

 anterior margin in a blunt, but distinct, angle. Posteriorly the dorsal margin passes 

 into the posterior margin in a rather sharp curve, forming a blunt angle. Posterior 

 margin descending obliquely, but very steeply, almost straight, and curving into the 

 lower margin, forming with it the broadly roimded posterior end of the shell. 

 Lower margin very little curved in the anterior part and sloping upward; posteriorly 

 more distinctly curved. Its lowest point (and the greatest height of the shell) at 

 about 57 pr. ct. of the shell (60 pr. ct. according to D'Orbigny's figure). In front, 

 the lower margin curves up into the anterior margin. The anterior end of the 

 shell is narrower than the posterior, thus producing the oblique shape. 



Valves very convex, chiefly so in the anterior part and backward to the pos- 

 terior ridge, and without any flattening upon the sides. Behind the posterior 

 ridge the shell is strongly compressed, so that the posterior slope and the ujjper 

 posterior angle appear almost alate. The most extreme anterior end of the shell 

 is also slightly compressed. Diameter 59 pr. ct. of the length (58 pr. ct. according 

 to D'Orbigny). Beaks inflated and swollen, incurved, projecting over the lunula, 

 but only slightly elevated above the posterior part of the upper margin. Lunula 

 distinct, short, triangular, about half as wide as long. 



Epidermis dull, with crowded, irregular, concentric lines, which fw^qucntly 

 become lamellar: j^robably, where not lamellar, they arc worn off, and originally 

 the whole epidermis was cloth-like. No radial sculj^ture visible. A radial rib 

 upon the posterior slope, but rather indistinct (much less distinct than in D'Or- 



