130 BULLETIN OF THE BUREAU OF FISHERIES. 



septa (interlamellar junctions). In addition, the ovisacs are closed above at the base of the marsupial 

 gill, thus forming a completely closed sac within each water tube. In one case {Strophitus) this sac 

 is again divided into secondary compartments. * * * This peculiar structure of the marsupial 

 gill is developed only in the gravid female, and is absent in the sterile (nongravid) female. These 

 characters are apparently connected with the prolonged breeding season, and the peculiar secondary 

 water tubes serve for the aeration of the embryos in the marsupium. 



In a preliminary announcement of his new system of the Unionidie (1910a) he 

 briefly stated this discovery in the following words: 



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 female. 



The statement of the presence of these lateral compartments of the water tubes of 

 the gravid female, made in this brief form and without illustrations, misled us and 

 seemed at that time not to be in accord with our own observations on the marsupium 

 of Alasmidonia, Anodonta, and Symphynota, ihree of the genera included by Ortmann 

 in the Anodontinae. We had, it is true, seen narrow slit-like spaces lying opposite the 

 outer and inner faces of the water tubes, which were evidently not blood vessels, as the 

 ostia opened freely into them. We interpreted them as differentiations within the 

 lamellae themselves and supposed that they were merely collecting canals into which the 

 ostia opened from the outside and which led by irregular apertures on the other side into 

 the water tubes, as our sections showed here and there interruptions (now known to 

 have a different significance) in the inner wall of these canals. It did not occur to us 

 that these might be the lateral divisions referred to by Ortmann, as, in the sections of 

 the marsupium in which we had seen them, they appeared so evidently to lie wholly 

 within the lamellae. 



We were, however, in error, and our failure to recognize that these were really 

 divisions of the water tubes was due to the fact that the sections studied by us were 

 taken from near the ventral border of the gill, where the spaces are much narrower 

 and more slit-like, and also to the fact that at that time we had not happened to see 

 the lateral divisions in the process of being cut off from the water tubes during the 

 early stages of gravidity. Thinking that Ortmann had made some mistake in his obser- 

 vations, we unfortunately published a note (Lefevre and Curtis, 1910a) to this effect 

 and stated that no such division of the water tubes in the three genera referred to was 

 present. A more careful examination of our material, however, and a study of mar- 

 supia at different stages of gravidity showed us that Ortmann was entirely correct, and 

 we wish to express our regret at the overhasty publication of our note. The true facts 

 of the case are as Ortmann has stated them to be, although he has only very briefly 

 described the method of formation of the secondary septa which divide the lateral 

 compartments from the central portion of the water tube in which the embryos are 

 confined. Speaking of the origin of the septa, he says (191 1, P- 293) : 



In specimens where the eggs begin to go into the gills, this structure (the lateral divisions of the 

 water tubes) is sometimes not developed, but it appears soon, and the epithelial folds, which form the 

 secondary septa within the water tubes, begin to grow into the lumen of the water tubes, and the folds 

 of the opposing faces of the two septa finally unite in the middle. The point of union (cross section of 

 the line of union) is often distinctly seen in sections. 



