ortmann: south American naiades. 



511 



shorter on the average (H. 59 to 63 pr. ct. of T>., while D. charruanus varies from 48 

 to 65 pr. ct.). In addition D. piceiis is not so subtrapezoidal in outline, but rather 

 subovate, which is brought about by a stronger curve of the lower margin, which 

 ascends much more distinctl.y posteriorly, so that the posterior end of the shell is 

 more elevated above the base-line. At the same time the posterior end is rather 

 blunt and rounded, and not so pointed as in D. charruanus. 



All other characters of the shell are similar to D. charruanus. The radial 

 sculpture of (he epidermis is moi-e distinct. The inside of the shell is also similar, 

 but the pseudocardinals are simpler, always elongate and compressed, and there is 

 always only one in the left valve. They are not much cut up, but only crenulated 

 and rugose. Nacre white, though one of my specimens has purplish blue in the 

 cavit}^, probably a discoloration. 



Beak-sculpture, as far as can be seen in smaller shells, similar to that of D. 

 charruanus. But there is a very small specimen, which is only 10 mm. long, among 

 them, which is doubtfully referred here. In this case the sculpture consists of 

 fifteen radial bars, of which the eighth and ninth miite at their lower ends. The 

 anterior bars are shorter, but there is not much diflference in this respect from the 

 posterior bars. The bars are rather sharp and fine, but those upon the umbonal 

 ridge are slightly thicker, and the two last, upon the posterior slope are very fine 

 and shorter. 



Measurements. 



No. 



Sex. 



Uruguayana. 



1 



2 



5 



4 



7 



Montevideo. 



I 

 Lea's fig. I 



Length. 



57 

 47 



Height. 



Diameter. 



Beaks. 



19 mm. =63 pr. rt. of L. 

 20.5 " =.59 

 31 •■ =61 

 30 " =.5S 

 38 '■ =59 



35.5 

 28 



= 60 



11..5 mm. =38 pr. ct. of L. 

 13 •• =37 

 21 " =41 

 20.5 '• =40 

 28 '■ =44 



at 



25.5 

 19 



=45 

 =40 



8.5 mm. 

 10 



15 

 1.5 

 18 



16 

 12 



=28 pr. 

 =29 

 = 29 

 =29 



= 28 



=28 

 = 26 



vt. of L. 



Remarks.—There is not the slightest doubt that my specimens represent the 

 U. piceus of Lea, but the question of its possible identity with D. charnmnu.s must 

 be left undecided, also that of other possible synonyms {lepklus Lea, and firmus 

 Lea). I cannot unite these for the jiresent on account of certain peculiar structures 

 in the anatomy, which will be pointed out presently. 



Anatomy. — I have the soft parts of one male, one barren female, and five 

 gravid females, three of the latter with eggs, and two with glochidia. but in one 

 only are the glochidia mature. 



The structure is entirely like tiuit of D. chtirruanus. With regard to the mar- 



