ortmann: south amekican naiades. 579 



are less distinct, and that the i^alpi appear a httle hirger, with longer posterior 

 margins, which are connected at the base. These differences, however, may be 

 due to preservation. 



The septa of the inner niai-supial gill also have; the characteristic swelling, 

 not so close to their insertion with the outci' lamina, but more toward the middle 

 of the septum. 



45. MoNOCONDYL^A MiNUANA D'Orbigny (1835). 



Section of (jills: Plate XLVIII, fig. 7. 



Monocondylcea mimtnnn D'Orbigny, 1843, p. 612, PI. 70, figs. 8-10; Corsi, 1901, 



p. 452, fig. 34; Simpson, 1914, p. 1388. 

 Unio minuanus Sowerby, XVI, 1868, PI. 91, fig. 497. 

 MonocondijUea pazii Lea, Obs., XII, 1869, PI. 36, fig. 88; Pilsbry & Rush, 1896, 



p. 81. 

 Monocondyla'a paraguayana Simpson, 1900, p. 911 (pro parte). 



Type-locality.— Canelon Grande and del Rosario, Banda Oriental, Uruguay 

 (Arroyo Grande flows North to Rio Negro; Arroyo Rosario flows South to Rio de la 



Plata). 



Other Localities.— Arroyo de las Vacas, Uruguay (Corsi); Rio de la Plata, 

 Colonia, Uruguay (Pilsbry & Rush, pazii). 



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

 Brazil (J. D. Haseman coll., February 5, 1909). Five specimens with soft parts, 

 males and females. Rio Jacuhy, Cachoeira, Rio Grande do Sul, Brazil (J. D. 

 Haseman coll., January 26, 1909). Five specimens with soft parts, males and 



females. 



Distribution.— Rio de la Plata and its tributaries in the Banda Oriental; Rio 

 Uruguay and Rio Negro drainages; and in Guahyba drainage in Rio Grande do Sul. 



Characters of the Shell.—^hcW moderately thick, rather small (maximum length 

 55 mm.). Outline subtrapezoidal, strongly oblique, rather elongated (height 67 

 to 72 pr. ct. of length). Valves very slightly gaping at anterior end. Dorsal 

 margin straight behind the beaks, or gently convex, much lower in front of beaks, 

 where.it may be straight or somewhat concave, forming with the anterior margm a 

 more or less distinct, obtuse angle. Posteriorly the dorsal margin passes into the 

 posterior margin in a more or less distinctly obtuse angle. Posterior margin ob- 

 liciuely descending, straight, or gently curved, passing into the lower margin in 

 a strong curve, which forms the rounded lower posterior end of the shell, not 

 much elevated above the base-line. Lower margin in its middle and anterior part 



