6 LILLIE. [Vol. X. 



solves out as development proceeds, finally disappearing entirely 

 towards the close of embryonic development. The eggs of 

 Anodonta are free from the first. 



I obtained most of my material from two ponds within five 

 or six miles of Wood's Holl. In one of these, Chiverick Pond, 

 I obtained Unio only ; in the other, Fresh Pond, Anodonta 

 only ; yet the ponds were within 500 or 600 feet of each other. 

 The first pond, however, possessed a softer muddier bottom 

 than the second, which was literally paved near the margin 

 with round stones as large as cobble-stones. No doubt the 

 different sets of conditions proved more favorable to the one 

 genus or the other ; but it can hardly be held that conditions 

 most favorable to the one genus were necessarily fatal to the 

 other nearly allied genus. The close proximity of the two 

 ponds in question makes it probable that an interchange of the 

 genera in question takes place at not infrequent intervals. 

 Indeed, I once found a sickly-looking Anodonta in the Unio 

 pond. It is therefore improbable that this condition was due 

 to a difference in the original stock of the ponds in question. 

 It seems probable that in small bodies of water, at any rate, 

 the two genera may prove mutually exclusive. 



An interesting difference between these two genera has been 

 commented on by Flemming (No. 13), Schierholz (No. 30), and 

 Carus (No. 11). This is, that, whereas all the Anodontas in 

 a given locality extrude their eggs (which are immediately 

 fertilized) at one or two different times, so that from a great 

 many individuals taken at the same time only one or two dif- 

 ferent stages of development are procured, the different indi- 

 viduals of Unio are fertilized at different times, so that a large 

 number taken at one time will yield several different stages 

 of development. It is, however, noticeable in the last case that 

 the embryos of one catch can always be grouped in five or 

 six stages. It is thus much easier to obtain a complete devel- 

 opmental series of Unio than of Anodonta, For this reason, 

 and also because the eggs of Unio are much more favorable 

 for the study of segmentation and for sectioning, I have worked 

 almost exclusively on this form. What I have to contribute 

 on Anodonta concerns the glochidium only. The unsegmented 



