ortmann: south American naiades. 505 



Brazil (J. D. Hascman coll., February 5. 1909). One specimen, male, with .soft 

 parts. Pond along Rio Negro, Santa Lsabel, Uruguay (J. D. Haseman coll., 

 February 11. 1909). Four complete shells, and five isolated valves; of two of 

 these soft parts, males. 



Distribution.- -Drainage of the Rio de la Plata from its mouth and its tributaries 

 in Uruguay and southern Brazil up to the Rio Paraguay in Paraguay. Also reported 

 as crossing the divide, and going into the headwaters of the Amazons in Bolivia. 

 Not known from the Parana above Corrientes. 



This is a species easily recognized by the elongated-subtrai)ezoidal shape, 

 with the upper and lower margins nearly ])arallel, by the anterior position of the 

 beaks, by the rather swollen shell and distinct (although rounded) posterior ridge. 

 The sides of the disk are rather flattened, and in this resi)ect this species resembles 

 the chilensis-group, intergrading with it to a degree. My specimens are somewhat 

 variable in shape, being longer or shorter. None of them shows the beak-sculpture, 

 since the beaks are badly eroded in all, except in the specimen from Uruguayana, 

 where they are only a little eroded, and consequently a little more elevated. But 

 here also no beak-sculpture can be seen. It must occupy onh^ a very short space 

 near the beaks, hardly more than 5 mm. The specimen from Uruguayana has the 

 nacre suffused with red (already mentioned by D'Orbigny). 



There is not the slightest question that U. acutirostris Lea is an old, much 

 eroded, and somewhat distorted specimen of this species. Haas has already sug- 

 gested this. 



Anatomy. — I have only the soft parts of male specimens, but Lea has already 

 described them in the case of his D. acutirostris, and has furnished at least some 

 information about the marsupium. 



Judging from my material, the anal opening is closed above without forming 

 a supra-anal. Closed part over five times as long as the open, the latter slit-like, 

 short, shorter than the branchial opening. Liner edge of anal indistinctly crenu- 

 lated. Anal and branchial separated by a solid bridge, running a certain distance 

 inward. Branchial with fine papillae, about three times as long as anal. Mantle- 

 edges not united in front of it. Palpi subtriangular, lower margins convex, pos- 

 terior margins connected for about one-fourth or one-third of their length. 



Gills long and rather narrow. Li their posterior i)art they are of equal width, 

 or the outer one is slightly wider; anteriorly the inner is much wider. The outer 

 is considerably narrowed anteriorly, its anterior end being situated near the highest 

 point of the line of the attachment of the mantle. The inner gill has a straight 

 margin in the middle, and anteriorly it is only slightly narrower, its anterior end 



