1884.] © Zoölogy. 1051 
chances of variation and selection forward into the adult stage. 
Thus animals which hatch out in a complete state will most 
probably suffer modifications of that state, and not of previous 
non-locomotive forms. Hence the origin of fresh-water inverte- 
brates is connected with the great movements which have affected 
the earth’s crust. The earliest well known lacustrine areas are 
those of the old red sandstone, in one of which we meet with the 
earliest known fresh-water mollusk, Anodonta jukesti (Forbes). 
The lakes of the Permo-Triassic period contributed additions to the 
fresh-water fauna of the globe. The Neritidz and Melaniide are 
So closely connected with them they may be regarded as their 
collateral or direct descendants, and thus may have originated in 
riassic lakes, but not earlier. Other genera probably arose at 
€ same time; the occurrence in Cretaceous deposits of Unio, 
Physa, Valvata and Limnea in the Nearctic, Palzarctic, and Orien- 
tal regions, suggests a high antiquity for these genera ; and they 
may have existed in Paleozoic times. The lakes of the Tertiary 
Period furnished probably further contributions to our fresh-water 
fauna, such as Lithoglyphus and Dreissena. Thus, existing fresh- 
SHELLs oF Anticostr.—When leaving Ottawa in the summer 
of 1883 to study the flora of Anticosti, Professor Macoun promised 
me that he would endeavor to make as complete a collection as 
Possible of the land and fresh-water shells of that little know 
island. The result of his labors is most gratifying, and : vt 
that they were energetically and intelligently directed. His col- 
