292 MEMOIRS OF THE CARNEGIE MUSEUM 
structure of these septa show that the latter is highly favorable to stretching, 
and while they stretch out, they become thinner, and the folds of the epithelium 
become smoothened and flattened (Compare Pl. LXXXVI, figs. 2 and 38, 8 and 
9, 14 and 15; and Pl. LXXXVII, figs. 2 and 3). After the discharge of the 
mature embryos (glochidia) the gill and its septa assume again their normal 
shape. 
This structural feature of the marsupial gill of the female is met with in all 
our Unionide, and in some it is the only one found. To the latter belong the 
following genera of Simpson: Quadrula, Pleurobema, Unio, and Tritogonia. But 
among these we observe a division into two groups. In some both gills, inner 
and outer, have the structure of marsupial gills, and become charged with eggs 
in the breeding season (See Pl. LXXXVI, figs. 2, 3, 4); in others it is only the 
outer gill which serves this purpose (See Pl. LX X XVI, figs. 6, 7, 10, 11, 12, 14). 
According to my investigations, in the following species both gills are marsupial: 
Quadrula subrotunda (Lea); Q. kirtlandiana (Lea); Q. rubiginosa (Lea); Q. trigona 
(Lea); Q. pustulosa (Lea); Q. lachrymosa (Lea) (according to material from 
Kansas); Q. metanevra (Rafinesque); Q. cylindrica (Say); Q. undulata (Barnes); 
Q. hippopea (Lea); Tritogonia tuberculata (Barnes). 
Only the outer gills serve as marsupia in: Quadrula tuberculata (Rafinesque) ; 
Q. pyramidata (Lea); Q. obliqua (Lamarck); Q. coccinea (Conrad); Q. cooperiana 
(Lea); Plewrobema esopus (Green); P. clava (Lamarck); Unio crassidens Lamarck; 
U. gibbosus Barnes; U. complanatus (Dillwyn); U. productus Conrad. 
The bearing of these facts upon the systematic arrangement will be discussed 
later. 
There are further structural modifications of the water-tubes found in an associa- 
tion of forms of Unionide, which apparently compose a natural group, and have 
been already recognized as such by Sterki. This group is composed of the genera 
Anodonta, Anodontoides, Symphynota, Strophitus, and Alasmidonta of Simpson’s 
“Synopsis.”” Of these I have investigated the following species: Anodonta cataracta 
Say; A. grandis Say; A. imbecillis Say; Anodontoides ferussacianus (Lea) (and 
var. subcylindraceus (Lea)); Symphynota compressa Lea; S. viridis (Conrad); S. 
costata (Rafinesque); S. complanata (Barnes); Strophitus edentulus (Say); Alas- 
midonta undulata (Say); Alasmidonta marginata (Say) (and the var. varicosa 
(Lamarck)); A. heterodon (Lea).* 
8The existence of the structures described in the following paragraphs has been denied by Lefevre and Curtis (1910). 
As I have pointed out, however, (Ortmann, 1910a), their own figures show them. These lateral water-tubes cannot be blood- 
vessels, for which they apparently have been taken by these two authors, and this is so easily seen (even macroscopically) 
that we do not need to discuss it any further. 
