ORTMANN: FAMILIES AND GENERA OF NAJADES. 275 
female (see Plate XVIII, fig. 4) are rather distant 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 very close, forming narrow 
water-tubes (see Plate XVIII, fig. 4). The epithelial lining of the latter 
water-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. 
Gravid females are not at hand. According to previous observations 
of other authors (Harms, 1908, p. 696, fig. 1, and 1909, pp. 322 and 334; 
Haas, 1910a, 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 W. Israél. 
Structure essentially identical with that of U. pictorum. No 
gravid females have been observed. The glochidia have been figured 
by Schierholz (1889, pl. 4, fig. 63). 
Unio crassus Retzius.” 
Many specimens have been studied, received from W. Israél 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).7! 
One male, and four females from Germany and Hungary have been 
sent to me by W. Israél. 
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. 
*1 This form is not batavus Maton and Rackett, 1907 (see Haas, 19104, p. 108, 
and I1910¢c, p. 167), but is surely batavus Lamarck, 1819, which name, consequently, 
cannot be used. As Haas (1910d, p. 62) has shown, the oldest name is musivus, 
Spengler, 1793. 
