loo PROOKKDINGS OP THE ACADEMY OF [1881. 



the time when a new growth comniences — when tlie shell which 

 is to be is liardly more tiian plastic membrane, not backed up 

 with a still'ening ol' lime, as it is after the fabric is perfected. 



In Mountain Lake, near San Francisco, a few miles west of 

 said city, curionsly distorted pond snails of the genus Physa 

 occur, which at one time, some years ago, excited attention. The 

 season of their growth is the summer, and its generative warmth 

 is accompanied with the trade winds, which blow across the lake 

 with considerable violence ; the plastic shells of the Ph3'^sa; are 

 forced against plants, chips and various fragments, odds and ends 

 atloat in or around the lake ; and the outer lip thus gets dented 

 and bent, giving a curious twist to many of the individuals. A 

 figure (128) illustrating a distorted specimen from the foregoing 

 locality is given by Mr. Binney in his L. and F. W. Shells of 

 North America. However, I have no reason to believe that this 

 deformit}^ is transmitted, as only a small proportion of the multi- 

 tude are affected. 



The specimens on which Mr. Ingersoll's species is based, were 

 found by him, as stated, in a snow-fed pond of small size, between 

 or among high cliffs. As before implied, the vacillations in plane 

 of coil may be owing to interruption of growth by recurring- 

 periods of hibernation, the characters in the environment, men- 

 tioned by Mr. Ingersoll, affording a reasonable solution of the phe- 

 nomena. Such ponds are subject to marked climatic contingen- 

 cies ; and sometimes, or rather in some years, their basins are 

 nearly pr quite dry — and again, fluctuations of temperature, 

 according to the volume of water, which is an important factor, 

 are far more critical in small ponds than in lakes or large bodies 

 of water, where the extremes of temperature, as well as other 

 conditions, as quality of water, are less variable or extreme. 



These two aspects of variation, bulging and irregularity in 

 coiling are exhibited with more or less frequency in all of the 

 larger American species, and in a greater or less degree, through- 

 out the entire area inhabited by Planorbis ; occurring oftener, 

 perhaps, among colonies which inhabit elevated stations, than 

 with those living at altitudes nearer the level of the sea. I am 

 of the belief, too, that these aspects of variation are less frequent 

 among colonies inhabiting southerly and semi-tropical regions. 



All of the variations referred to are, when present, more con- 

 spicuous in the larger forms west of the Rocky Mountains, for 



