474 MEMOIRS OF THE CARNEGIE MUSEUM. 



Other localilies. ^Y on Ihvr'mg, (1S93, p. 119) gives this species from Rio Para- 

 guay, but also upon D'Orbigny's authority/^ from the Amazon-drainage. Von 

 Martens (l. c.) reports it from Paraguay. No other locahties have been hitherto 

 cited. 



New locality. — Rio Paraguay, Sao Luiz de C'aceres, Matto Grosso, Brazil, 

 (J. D. Haseman coll., May 25, 1909). One specimen. 



Distribution. — The n(>w locality is not very far from the type-locality (Rio 

 Tacaraca), about 400 kilometers up the river, and demonstrates that this species 

 belongs to the upper Paraguay drainage in Bolivia and western Brazil. According 

 to Von Martens it goes down this river to Paraguay. 



D. hylmis and guaranianm have been united by Lea, Sowerby, and Simpson, 

 but 1 think that this is not correct, and that I have both species before me, agree- 

 ing well with D'Orbigny's remarks about them. The real D. hykcus is a larger 

 shell, with the lower posterior end somewhat more produced, and with differences 

 in the beak-sculpture, which extends over a larger section of the shell, and con- 

 sists of rounded bars, which are somewhat irregular, and are rugose or slightly 

 nodulous. The figure of D'Orbigny (Fig. 8) shows the sculpture very well. 



I have only a single individual of this species, without soft parts, and have 

 drawn the following description from this. 



Description of Shell. — Shell comparatively small, moderately solid, subovate 

 or subtrapezoidal, slightly higher behind. Height 64 pr. ct. of length. Dorsal 

 margin very gently curved, passing into the posterior margin in a blunt, rounded 

 angle. Posterior margin obliquelj^ descending, emarginated, but this emargina- 

 tion undoubtedly is an individual feature, since the growth-lines indicate that it was 

 not present, when the shell was j'ounger. Lower posterior angle slightly pro- 

 duced, but rounded. Lower margin with its lowermost part placed at between 

 two-thirds and three-fourths of the length of the shell (from anterior end), curving 

 up behind. In the anterior portion it slopes upward in an almost straight line, 

 finally cm-ving up into the anterior margin ; thus the shell appears slightly narrower 

 in front than behind. 



Valves rather flat (in this respect my specimen differs from the original hyhcus, 

 which is more convex), gently and rather uniformly convex, with the umbonal 

 (posterior) ridge weakly marked, and indicated chiefly by a shallow radial de- 

 pression upon the posterior slope, which forms the emargination at the posterior 

 margin, and makes the posterior ridge appear slightly biangulate towards the 



'- lie says that, all forms rcportcil liy D'Orbigny ridiii l?(ilivia ai'c fniiu the Aiiiazon-dnuiiage. In 

 this i)articular case tliis is not correct, since the Rio Tiicaraca of Boli\ia drains into the Paraguay. 



