ortmann: south American naiades. 615 



All of these previous measui-eiiieuts fall within llic range of variation of my 

 series, with the exception of the diameter given by D'Orbigny, which is somewhat 

 lower (33 pr. ct.) than any of my figures (lowest 36 pr. ct.). 



Remarks. — This species is very variable in shape, being sometimes higher, 

 sometimes more elongated. There is no indication that the shape is connected 

 with sex. Since no very young specimens are at hand, and never have been ob- 

 served, we do not positively kno\v anything about their shape, but from the growth- 

 lines of the old specimens it is seen that the young shell generally must have been 

 comparatively higher: the shell grows, with advancing age, more in the direction 

 of the posterior end, so that the longitudinal diameter increases more than the 

 vertical. 



There is no doubt in my mind that A. crassa and wyniani belong here, and that 

 they represent specimens which are a little more elongated than the average; but 

 among the set from San Isidro, I have specimens which represent even greater ex- 

 tremes than these. 



A. patagonka is rather thick and solid, hut varies also in this respect. The 

 specimen from Montevideo at hand is not quite as thick as the others, but agrees 

 with them in other respects, being also rather elongated. Just such specimens 

 induce me to regard the next form (rubicunda) as a variety of patagonica. 



Anatomy. — Soft parts of eight males, one barren, and eleven gravid females 

 have been investigated. The breeding season is January. 



The anatomy of hilomarginata and wymani has been previously described by 

 Lea (Obs. X, 1863, pp. 391, 394) as far as the superficial characters of the gills, 

 the palpi, and the branchial and anal openings are concerned. 



Von Ihering (1891, p. 480) describes the eggs and lasidia of Glabaris wymani, 

 the former being 0.09 mm. in diameter, the latter 0.086 mm. long. However, since 

 he says that his specimens are from Rio ( 'amaquam, I am not sure that they have 

 been properly identified. They might belong to A. iheringi (Clessin). 



Anal opening entirely open, separated from the branchial by a connection of 

 the mantle-margins. Inner edge of anal smooth above, very finely crenulated 

 near the lower end. Branchial opening with small papillae. Palpi nearly semi- 

 circular, longer than liigh, behind abruptly truncated, the posterior margins con- 

 nected at base. 



Gills rather wide, the inner much wider than the outer in front, its anterior 

 end immediately behind the palpi, and attached along the whole space between 

 the pali)i and the anterior end of the outer gill. Inner lamina of inner gill entirelj- 

 cormected with abdominal sac. Septa of the gills well developed. In the non- 



