ortmann: south American naiades. 573 



The following key is intended only for the distinction of the forms at hand. 



Key to the Species of Monocondyl.ea. 

 tti. Shell h^iiKill, or of incdiiini size (at the utmost 50 to 60 mm. long). Lamellar stria; of epidermis well 

 developed, but sometimes abraded. Color of epidermis greenish black or brown. 

 hi. Shell more or less oblique, suiitrapczoidal, or angularly suborbicular, witii a well developed 

 posterior upper angle, high behind, narrower in front. Color of epidermis greenish bUick. 

 Prismatic zone on inside of margin of unequal width, very wide along anteri<ir portion of 

 lower margin. 

 r,. Shell angularly suborbicular, rather high and short, height about SO pr. cl. of lengtli. Sides 

 of disk not flatteneil. 

 (/i." Shell compressed, ilianieler .")0 pr. ct. or less of length. Beaks not much elevated. 



M. lenli'fortnis. 

 <1'2- Shell mure swollen, dian)eter 58 to 5!) pr. ct. of length. Beaks more inflated and 



elevated M. paraguayana. 



c<. Shell subtrapezoidal, more elongated, height about 70 pr. ct. of length. Sides of disk more 

 or less flattened. 

 di. Shell more swollen, beaks more prominent, hinge-line rather strongly incurved under 



the beaks -V. iiiinuana. 



d-i. Shell less swollen, beaks less prominent, hinge-line less incurved under the beaks. 



.1/. minuana parchappi. 



hi. Shell very little, or not at all oblitpie, subovatc or subelliptical in outline, with the posterior 



upper angle rounded and poorly developed. Obesity of shell very great, and beaks much 



swollen and inflated. Color of epidermis brown. Prismatic zone on inside of margin narrow 



and of nearly uniform width ^I- obesa. 



a-.. Shell large (about 100 mm. long). Lamellar stria' of epidermis poorly developed upon the disk, 

 distinct only near the margins. Color of epidermis from yellowish olive upon the disk and toward 

 the beaks to dark brown toward the margins M. Iwllandi. 



The species of this genus cover a rather large range in South America being 

 found all over the La Plata-drainage, in the Amazon-drainage in Bolivia and 

 Brazil, in the Rio San Francisco in eastern Brazil, and also in coastal streams in 

 southern Brazil (Rio Grande do Sul). 



43. MoNOCONDYL^A LENTiFORMis Lea (1866). 



Anatomy of gills: Plate XLVII, fig. 3. 



Monocondijkca lentiformis Lea, Obs., XII, 1869, PI. 36, fig. 86; Pilsbry & Rush, 



1896, p. 81; Simpson, 1900, p. 912; 1914, p. 1392; Corsi, 1901, p. 452; Haas, 



1916, pp. 25, 54. 

 Aplodon lentiformis Von Ihering, 1893, ]). 67; Nehring, 1893, p. 164. 



Type-locality. — South America. 



Other Localities. — Rio Piracicaba, Piracicaba, Sao Paulo, Brazil (Von Ihering) 

 (Nehring) ; Rio Uruguay, Salto Oriental, Uruguay (Haas) ; Rio dc la Plata, Colonia, 

 Uruguay (Pilsbry & Rush). 



