BIOLOGY OF PHYSA 13 



below this part of the creek, devoid of snails. The stones might 

 be removed by man or by the stream and redeposited farther 

 down its course during the spring floods. The deep pool lying 

 above the optimum habitat, Fig. 3, might also become filled up 

 so that the silt could no longer be deposited in its quiet depths 

 and in consequence the silt from up stream would be carried 

 past such a habitat. When such a deposition of silt occurs 

 over a region it will then show similar physical conditions to 

 those found in Fig. 4. Just what would 'be the result of a silt 

 deposit upon the habitat where Cladophora has so strong a foot- 

 hold would be difficult to say. It does not seem probable that 

 under ordinary circumstances the stream would be carrying 

 enough silt to kill the plant, but during the heavy rains the 

 stream might carry a load sufficient to scour the stones and thus 

 dislodge or kill the plant. This might furnish a possible explan- 

 ation of why Cladophora is found in the region of Fig. 3, and 

 not in that shown in Fig. 4. 



It is interesting to note in connection with this so-called 

 creek-bed optimum that Baker (1901 p. 660) cites conditions 

 very similar in the Genesee river, 



"Below the dam, which is now broken and very much out 

 of repair, the river flows rapidly over a rocky bed, in many 

 places the water being so shallow that persons can wade across. 

 The right bank is very rocky, more so than the left, and forms 

 little sheltered bays here and there, in which Goniohasis and 

 Physa live by thousands, the former making a veritable pave- 

 ment in some places." 



He goes on further to say that near the railroad bridge where 

 the water becomes deeper, the number of Physa decreases in 

 number and that in deeper water still they disappear altogether. 

 He cites another place in the river as follows : ' ' The walls are 

 very steep and stony, the river having cut away the soft shale, 

 leaving great projecting ledges. Physa, Planorhis and Lymnaea 

 are here notably abundant, living on ledges of rocks where 

 the water is a few inches deep." 



As was mentioned above, Physa was not found in the Huron 

 river either in the vicinity of Ann Arbor or Ypsilanti except 

 a few young ones found living upon water weeds in slack water. 

 It was thought, perhaps, that this scarcity of snails is due 

 to insuffipent protection from current, especially during high 



