504 MEMOIRS OF THE CARNEGIE MUSEUM. 



4. Group of Diplodon charruanus. 



Shell not compressed, but rather evenly convex and often considerably swollen 

 when old. Outlme subelliptical or somewhat subtrapezoidal, elongate, more 

 or less pointed behind, straight, and not oblique. Beak-sculpture simple, with 

 narrow, straight, uninterrupted bars, restricted to the region of the beaks. A 

 few of these bars may be joined at their lower ends. 



The chief character by which this grouj) is distinguished from that of chUensis 

 is the absence of a flattening of the valves upon the sides. The valves are generally 

 evenly convex, and in consequence of this, at least in larger specimens, the shell 

 appears as more swollen. In other respects, the shelbis similar in outline and other 

 characters, except that the posterior end is often produced into a distinct blunt 

 l^oint, which may be more or less elevated above the base-line. There is no dis- 

 tinct tendency of the posterior portion of the shell to be higher than the anterior, 

 and thus the shell is not oblique. A posterior ridge may be distinct or indistinct. 



The first species, D. parallelopipedon, (Lea) froms a transition toward the 

 group of chilensis. 



The metropolis of these forms is in the sj'stem of the Rio de la Plata, but they 

 are also found in the coastal streams of southern Brazil (Rio Grande do Sul). 



13. Diplodon parallelopipedon (Lea) (1834). 



Unio parallelipipedon D'Orbigny, 1843, p. 609; Corsi, 1900, i). 447, fig. 30. 



Diplodon parallelipipedon Simpson, 1914, p. 1275. 



Unio parallelopipedon Von Martens, 1894, p. 164; Pilsbry & Rush, 1896, p. 30. 



Diplodon acutiroslris (Lea) Simpson, 1914, p. 1276. 



Diplodon parallelipipedon and acutiroslris Haas, 1913, pp. 22, 23, 52, 53. 



Type-locality . — Rio Parana, Province of Corrientes, Argentina. 



Other Localities. — ^Arro.yo del Rosario, Uruguay (to La Plata) (D'Orbigny); 

 Arroyo de las Vaccas, LTruguay (Corsi); Rio de la Plata, Colonia, Uruguay (Pilsbry 

 & Rush); Rio Uruguay, Salto Oriental, Uruguay (Haas); Swamps of Rio Parana 

 from Buenos Aires to above Corrientes, Argentina (D'Orbigny); Rio Paraguay 

 (Von Ihering, 1893, p. 119); Paraguay (Von Martens); Rio de San Miguel, Prov. 

 of Chiquitos, Bolivia, (D'Orbigny). 



The last locality deserves special attention, since it is in the Amazons-drainage; 

 but it is in the most southern extremity of it, close to the divide toward the Para- 

 guay. It certainly requires confirmation. 



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



