138 BULLETIN OF THE BUREAU OF FISHERIES. 



lost in Obliquana as it has been in Strophitus, and that the metamorphosis takes place 

 while the glochidia are in the conglutinates ? We have not yet had the material by 

 which to answer this question. 



The relation of the embryos and glochidia of Strophitus to each other is so unusual 

 that its description is reserved for a special section (see below). 



STRATinCATlON OF UNFERTILIZED EGGS. 



It has alreaay been pointed out that not infrequently eggs pass into the marsupium 

 without being fertilized and remain there throughout the period of embryonic develop- 

 ment, as one may find them in the same gill with fully formed glochidia. In some 

 individuals we have found every egg in the marsupium in this condition.. Such eggs 

 have been encountered chiefly in summer-breeding species, and they seem to be especially 

 common in Pleurohcma and Qnadrula, nearly every gravid female of which has been 

 found to contain at least some unfertilized eggs. After remaining in the marsupium 

 for a time such eggs generally become swollen and stratified into three distinct layers, 

 a heavier, often pigmented, mass at one pole, a clear or hyaline intermediate zone, and a 

 small granular cap at the lighter pole. As the eggs lie in a constant position in the 

 gills, which are placed vertically in the normal position of the animal, it can not be 

 doubted that the stratification is produced by gravity. It has not yet been determined 

 whether the substances which occur in these layers are the same as would be separated 

 out by centrifuging or not, but this is not at all unlikely. As many of the species of 

 mussels in which we have seen this condition, for example, Qnadrula ehena, Q. trigona, 

 and Pleurobema asopus, have brightly colored red or pink eggs, the stratification is 

 quite striking, the pigment being always at the heavier pole, as it is invariably directed 

 toward the lower border of the gill. 



ABORTION OF EMBRYOS AND GLOCHIDIA. 



There has been a certain amount of discussion among the conchologists as to 

 whether or not the functioning of all four gills as a marsupium is a constant character 

 in Qnadrula, and observations have been to a certain extent conflicting. Since Simpson 

 has made use of this feature in characterizing the group Tetragena, some importance 

 has been attached to the apparent discrepancy in observations. 



While examining mussels on the upper Mississippi River in the summer of 1908, 

 we observed a peculiarity of behavior in all of the species of Qnadrula collected which 

 may account for the conflicting descriptions of the marsupium in this genus, and also 

 for the fact that in some species gravid females have never been observed at all. Every 

 species of QuadriUa that came into our hands exhibited to a greater or less degree the 

 habit of aborting embryos and glochidia when taken out of the river, and if they were 

 not opened and examined at once upon capture they were generally found shortly 

 afterwards to be either partially or entirely empty. Some individuals discharged the 

 contents of their gills more readily and completely than others, the abortion involving 



