Ortmann; Families and Genera of Najades. 276 



female see Plate XVIII, fig. 4) are rather distanl from each other; 

 in the outer gill of the male, they are slightly more crowded. But 

 in the female the septa of the outer gill are verj close, forming narrow 

 wdter-tubes (see Plate XVI 1 1, fig. 4). The epithelial lining of the latter 

 Wciter-tubes is marsupial in character. Thus the marsupium is formed 

 by the outer gills alone practically throughout their whole extent; 

 for small sections at the anterior and posterior ends of the gill, which 

 have wider water-tubes, pass gradually into the marsupial part, and 

 may be disregarded. 



( rravid females are not at hand. According to previous observations 

 of other authors (Harms, 1908, p. 696, fig. I, and 1909, pp. 322 and 334; 

 Haas, 19100, p. 107), the glochidia are subtriangular, with hooks. 

 Size 0.29 mm. 



Unio tumidus Retzius. 



The soft parts of four males and six females from Germany and 

 Hungary are at hand, received from YV. Israel. 



Structure essentially identical with that of U. pictorum. Xo 

 gravid females have been observed. The glochidia have been figured 

 by Schierholz (1889, pi. 4, fig. 63). 



Unio crassus Retzius. 20 



Many specimens have been studied, received from W. Israel from 

 the drainage of the river Elster in Thuringia, Germany. 



In this species also the soft parts are identical with those of U. pic- 

 torum. The non-marsupial part at the anterior and posterior ends 

 of the outer gills of the female is sometimes a little larger, but there 

 are specimens exactly like U. pictorum in this respect. Glochidia 

 have not been observed, and have not been described so far as I know. 



Unio crassus musivus (Spengler). 21 

 One male, and four females from Germany and Hungary have been 



sent to me by W. Israel. 



Absolutely identical in structure with U. crassus. A female from 



the river Begas, Hungary, had a few ovisacs near the middle of the 



20 For nomenclature, see Thiele, 1909, p. 35. 



21 This form is not batavus Maton and Rackett, 1907 (see Haas, 1910a, p. 108, 

 and 1910c, p. 167), but is surely batavus Lamarck, 1819, which name, consequently, 

 cannot be used. As Haas (19100", p. 62) has shown, the oldest name is musivus, 

 Spengler, 1793. 



