ortmann: south American naiades. 



513 



which are more lamellar and compressed in the latter (a smaller shell), and more 

 stumpy and rut up in apprimiis. My material shows that the character of the 

 hinge-teeth changes with age. In general my specimens correspond in size and 

 shape to urugnayensis, luit the larger ones have more stumpy teeth, and thus I 

 believe that apprimus is an old specimen of uruguayensis. Both Simpson and 

 Haas (1916, j). 12, 47) unite apprinms with wymani, which cannot be correct on 

 account of the difference in obesitj'. I think wymani belongs to lacteolus (See 

 below). 



However, it is quite possilile that all these forms are variations of one and 

 the same species, and then, of course, our general arrangement must be changed. 

 As regards the present form I can only say that it looks like a very large and heavy 

 charruanus, the shell being rather elongated, subtrapezoidal, and much swollen. 

 In lacteolus, the shell is higher and more ovate, and much more compressed. In 

 none of my specimens is the beak-sculpture seen. 



Measurements. 



Anatomy. — Soft parts of a male and a barren female at hand. 



Color of soft parts: distal part of foot dark gray, this color sharplj' marked off 

 from the whitish basal part. 



Anal closed above; closed part about four times as long as the open part, 

 which is slit-like, and shorter than the branchial, and separated from it by a solid 

 mantle-connection. Branchial opening with small, but distinct, paiiilhe. Palpi 

 rather large, subtriangular, with lower margins strongly convex; posterior margins 

 connected at base. 



Gills long and moderately wide, the inner one much wider than the outer 

 anteriorly, its anterior end immediately behind the jialpi. Inner lamina of inner 

 gill entirely connected with alxlominal .sac. Non-marsupial gills with scattered, 

 short, interlaminar connections. In the female a large section of the inner gill 

 is marsupial, about one-fourth of the gill remaining non-marsupial at the anterior 



" The measurements of vrtiguayensis and apprimus are those given by Simpson for tiie respective 

 tj'pes. Those for apprimus are given by him uiuler irt/mani (p. 1231). 



