GEORGE LEFEVRK AND WINTKKTnX C. CL'RTIS. 



tiiuc: "Water-tubes in the gravid female divided longitudinally 

 into three tubes, one lying toward each face of the gill, the third 

 in the middle; only the latter contains eggs or embryos, and is 

 much larger than the other tubes. This division into three parts 

 is not present in the sterile (sic] female." Although it is not 

 specifically stated, it is to be inferred from the above description 

 that the divisions of the water-tubes into three parts is due to 



T- .-; 



' .' c; 



FIG. i. Anodonta cataracla Say. Hori- 

 zontal section of portion of gravid mar- 

 supium, showing a water-tube, undivided 

 and filled with embryos. O.L, outer 

 lamella of gill; I.L, inner lamella; I.J, 

 interlamellar junction; W.T, water tube; 

 E, embryos, X3I.5 Kline del. 



FIG. 2. Alasmidonla truncata 

 Wright. Horizontal section of por- 

 tion of gravid marsupiiim. showing 

 a water-tube, undivided and tilled 

 with embryos. The mass of em- 

 bryos is somewhat contracted into 

 the middle of the tube. X3I.5 

 Kline del. 



the presence of longitudinal partitions running parallel \\ith the 

 lamella 1 , but no intimation is given as to how they arise, <>t how 

 they disappear after the marsupium has discharged its contents. 

 To any one familiar with the structure of the gills of the I 'nionidoj 

 the statement that the water-tubes exhibit a temporary division 

 into three parts is on its face improbable, for it would be difficult 

 to imagine how such a division could be brought about, and still 

 more difficult to understand why, when once established, it 



