ortmann: south American naiades. 



545 



diictor-scar faintly impressed, almost pear-shajK-d, subovate or subtrianj^ular, 

 with an upi)er trianjj;ular i)roccss formed by the posterior retractor-scar. Mantle- 

 line distinct, remote from the margin aljout one-fourth to one-fifth (or less) of the 

 height of the shell. Dorsal scars about five (more or less), in beak-cavity in an 

 irregular, longitudinal row, sometimes shifted a little toward the narrow hinge- 

 plate. 



Measurements.^' 



Remarks. — Lea has described two other large, subrotund species from the 

 Uruguay River: nocturmis and funebralis (Obs. X, 1863, PI. 42, fig. 288, and PI. 41, 

 fig. 286). Of these, funebralis is similar in general shape and proportions, with 

 the exception of the much greater compression of the valves. The diameter is 

 only 30 pr. ct. according to the figure and the measurements given by Simpson 

 (1914 p. 1284), which falls far below of any of the measurements known for paranen- 

 sis. In addition, in funebralis, the mantle-line is unusually far remote from the 

 margin of the shell, and, according to our present knowledge, we must consider 

 these characters as sufficient to separate the two species. 



U. nocturnus was united with paranensis by Simpson in 1900, but in 1914 

 he separated them again. The proportions of nocturnus are: 



Length. 



Height. 



Diameter. 



Beaks. 



Lea's I 



figure. . .72 mm. 60 mm. =8.3 pr. ct. of L. .30 mm. =41 pr. ct. of L. at 21 nmi. =29 pr. rt. of L, 

 Simpson's! 1 



measur. . 72 " 60 " =83 " ,28 " =39 



Greatest Height. 



at 37 mm. =51 pr. ct. of L. 



Thus there is no essential difference from paranensis, and according to Simp- 

 son's account, I can find only a difference in the color of the epidermis, which is 



-' More care must he tak(>n in this species than in others to phice the ligament horizontally, and to 

 measure parallel and vertical to it. The greatest height of the siioil is hard to locate, and allowance 

 should be made for this. 



