ortmann: south American naiades. 



617 



Closely allied to .-1. patngonica, but differing in being somewhat smaller (maxi- 

 mum length 84 mm.), with a thinner shell. The outline of the shell is more nearly 

 and more frequently subrotund, and the diameter ranges higher (up to 50 pr. ct.). 

 The nacre very often, but not always, has reddish or i)urple tints. 



Older Measurements. 



Simpson, rubicunda. 

 Do. 



Simpson, fuizi 



Do 



PiLsbry, felix 



Do 



=86 

 = 79 

 = 72 

 = 72 

 =71 

 =72 



30 

 36 

 26 

 26 

 18 

 20.5 



=51 

 = 48 

 = 43 

 = 49 

 = 37 

 =39 



(Type) 



(Type) 



Remarks. — All the previous measurements fall within the range of variation of 

 my specimens, only the type of Lea's rubicunda is higher, shorter, and more swollen 

 than any of my specimens. Thus we are to regard the type of rubicunda as an 

 exceptional specimen, which is also peculiar in the fact that the posterior margin 

 forms nearly a right angle with the upper margin. All these peculiar features are 

 correlated, however. The normal type of this form is better represented by Lea's 

 pazi; yet there are specimen in my material which distinctly approach the ruhicunda- 



type. 



It is unknown whether this form ever reaches the size of A. patayonica, and 

 in the absence of young specimens of the latter it is impossible to compare them 

 directly. But it seems that the more nearly rotund shape is rather a juvenile 

 character, although there are young specimens in my material which are more 

 elongated. From the measurements given it is seen that the height ranges from 

 70 to 84 pr. ct. in my material, reaching 86 pr. ct. in Lea's type of rubicunda, while 

 it ranges from 60 to 74 pr. ct. in patarjonica. The diameter in rubicunda is from 37 

 to 50 pr. ct. (51 pr. ct. in the type), while in patagonica it is from 36 to 44 pr. ct. 



Clearly height and diameter are correlated, a shorter and higher shell being 

 also more .iwollen than a more elongate one. The slight difference in the location 

 of the beaks (30 to 39 pr. ct. in rubicunda, 28 to 34 pr. ct. in patagonica) is also 

 connected with this. Finally it may be that all these differences in the shape are 

 connected with age, and this may be true also of the thickness of the shell. How- 



