Ortmann: Families vnd Genera oi Najades. 237 



thai ii is oi prime significance. In all my investigations I have never 

 come arid-.- an exception or variation in the marsupium. It is i rue thai 

 in species which have normally all four gills marsupial, sometimes only 

 two gills are found charged, lint all authors, who record such cases, 

 only mention the fad thai the gills were charged, withoul saying any- 

 thing aboul the structure of the gills. I have also met with such 

 cases: bul invariably a closer investigation revealed the fact thai 

 the other gills which were not charged also possessed marsupial 

 structure, and consequently were capable of being charged with eggs. 



On the other hand, in those cases, when the outer gills alone serve 

 as marsupium, it was not the simple fad thai the} alone were filled with 

 eggs in the breeding season, which was ascertained. It was the in- 

 vestigation of the structure' of the gills, which induced me to judge 

 the character of the marsupium. Lefevre and Curtis (1910, p. 83) 

 are inclined to regard my observations in Pleurobema coccineum as 

 due to accidental conditions. But this is surely not so. I have seen 

 now a great number of individuals of this species with the outer 

 gills alone charged, and I have seen and examined many more, females 

 in the sterile condition, which invariably had marsupial structure 

 only in the outer gills, while the inner gills were different, and not 

 built to receive ctrgs. Not a single exception was observed. 



Further it is quite evident that the arrangement of four gills serving 

 as marsupia is found in a number of groups, the species of which are 

 undoubtedly closely allied. This is clear in the peculiar Quadrula 

 piicata-gr oup, in the metiuicvra-group and others. Then again, a 

 marsupium formed by the outer gills alone is characteristic of other 

 natural groups. To me the most interesting case was that of Pleuro- 

 bema coccineum. Here I discovered first that this supposed Quadrula 

 differs from the Quadrula-type; I also discovered that this species 

 intergrades with Q. obliqua and Q. pyramidata. This being the case, 

 I concluded that the latter also should have a marsupium like coc- 

 ci nea. And this proved to be true! 



Nevertheless the character of the marsupium should not be too 

 implicitly relied upon. There is no question that the condition in 

 which the four gills serve as marsupia is more primitive than the stage 

 where only the outer gills are marsupial. 8 Hut it seems to me that 



8 The functional and morphological progress from the four-gill-marsupium to the 

 t\vo-xi!l-marsupium has been correctly understood and expressed l>y Haas (,19101', 

 p. 19). 



