ortmann: south American naiades. 553 



margins long and strongly curved, posterior margins short, and united at base for 

 about one-fourth of their length. 



Inner gills much wider than the outer, chiefly in front; only for a short 

 distance behind have they an equal width. Outer gill with curved margin, anterior 

 end at highest point of mantle-attachment-line. Imier gill with margin nearly 

 straight, broadly attached anteriorly, and anterior end close to the palpi. Inner 

 lamina of inner gill entirely connected with abdominal sac. Structiu-e of non- 

 marsupial gills normal, with scattered interlaminar connections, assuming here and 

 there the shape of short, weak septa. 



• 



Genus CA8TALIA Lamarck (1819). 



Castalia Lamarck, 1819, p. 66; Pilsbry, 1911, p. 610. 



Tetraplodon Spix, 1827, PI. 25; Pilsbry, 1893, p! 90; Simpson, 1900, p. 863; 1914, 



p. 1194. 



Pilsbry (1911) has brought to light the fact that Savigny's genus Castalia 

 (Vermes) is not to be dated from 1817, but that it is later (certainly not before 1820, 

 probably as late as 1826), so that Castalia applied by Lamarck (1819) to the present 

 genus has to stand. 



Castalia is distinguished by the triangular outline, with high beaks, well de- 

 veloped posterior ridge, great obesity of the shell, truncated posterior slope, and 

 well developed beak-sculpture, which extends over a large part of the shell. It 

 is to be noted, however, that the truncation of the posterior slope often varies with 

 age, so that in very young specimens it is somewhat elevated in the middle, giving 

 an outline to the shell approaching the subtrapezoidal or subrhomboidal. With 

 one exception the species resemble each other very closely. This exception is 

 C. undosa, which differs from all the rest in having strong, oblique folds, or ribs, upon 

 the posterior slope. 



My material of Castalia is rather insufficient, and I have had great difficulties 

 in determining the species. The key given by Simpson (1914) is useless for the 

 reason that the type-species {ambigua) has been misunderstood. The latter has 

 been redescribed and figured by Von Ihering (1910),-^ which fact had been neglected 

 by Simpson. At the same time Von Ihering has given an elaborate key for the 

 species. However, this also is unsatisfactory, and I am afraid that Von Ihering 

 has not paid proper attention to the changes midergone by the shell with advancing 



^ In this connection it should he nieulioned that Wyiitt, T. (.\ Manual of Conchology, 1S3S, p. 6.5, 

 PI. 11, fig. 5) ha.s given a recognizable figure of C. ambigua, which unilouhteilly represents the genuine 

 ambigua, as determined by Von Ihering). 



