ortmann: south American naiades. 517 



rim around the anterior-lower-postorior margin, representing, apjiarenth', the first 

 rudiments of the postembryonal shell. Size (without' rim) : L. 0.29 to 0.30; H. 

 0.26 mm.; (with rim): L. 0.34 to 0.35; H. 0.28 to 0.29 mm. 



The hooklcss gloehidium, i)rovided with a rim or margin, i.s higiily interesting 

 in view of the fact, that this structure has not been observed in other species of the 

 charruanus-group. 



5. Group of Diplodon lacteolus. 



Like the fourth group (that of charruanus), but shell higluu- and shorter, 

 subtrapezoidal to ovate, chiefly so when \'oung, comiiressed or somewhat swollen. 

 Beak-sculpture fine or a little heavier and better developed, but not covering a 

 large part of the shell. 



This group stands close to that of D. charruanus. The greater height of the 

 shell is chiefly evident in the young shell, which may be slightly oblique. But 

 the older shells are also shorter and higher than in the species of the churruanus- 

 group, although they generally are less elevated than younger shells. They are 

 not oblique, and have a rather regular, broadly ovate, or subelliptical outline. 



. 18. Diplodon burroughianus (Lea) (1834). 

 Anatomy of gills: Plate XLVI, fig. 4. 



Unio burroughianus Lea, Obs., I, 1834, PI. 10, fig. 27; D'Orbigny, 1843, p. 609; 



Von Martens, 1894, p. 164; Corsi, 1901, p. 450. 

 Diplodon burroughianus Simpson, 1914, p. 1271. 



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



Other loculiti es.Smull rivers of the Banda Oriental in Uruguay (D'Orbigny) ; 

 Montevideo (D'Orbigny); swamps along the Parana from Buenos Aires to above 

 Corrientes (D'Orbigny); Paraguay (Von Martens). 



It also occurs near Santa Cruz de la Sierra in Bolivia (D'Orbigny) which is 

 in the drainage of the Amazons. 



New Locality.— Fond near the Rio Negro, Santa Isabel, Uruguay (J. D. Hase- 

 man coll., February 11, 1909). Three complete specimens, two of them, male 

 and female, with soft parts, and four odd valves. 



Distribution.— Brahmge of Rio de la Plata, including the small streams of the 

 Banda Oriental, Rio Negro, and the Paraml and Paraguay to Paraguay, and possibly 

 also in the Amazon-drainage in Bolivia. The latter i)arf of the range, however, 

 should be confirmed. 



Some of my specimens, chiefly the female with soft parts, agree very well with 



