BIOLOGY OF PHYSA 17 



The follo\ving conditions prevail in this habitat: Shallow 

 water, from five to eight inches deep ; full sunlight ; a small amount 

 of algae, but enough for food; clear sandy substratum; no vege- 

 table debris; fresh water supply without severe wave action; 

 and an absence of enemies, so far as observed. 



Another class of beach pool habitats is furnished by small 

 sedge-filled shores of small bodies of water, such as is found 

 in the pond in Snow's swamp at Kavanaugh Lake. Some pres- 

 sure was brought to bear upon the spongy shore, causing a 

 depression which the water from the lake quickly filled. Refer- 

 ence to the preceding diagram, Fig. i, will show the presence 

 of Physa gyrina in such a habitat. 



b. Physa Habitat Behind an Ice-Rampart. Kavanaugh Lake 

 lies in Washtenaw County, Mich., about 22 miles west of 

 Ann Arbor. It is about one mile long and one-fourth of a mile 

 wide and is surrounded by sandy morainic hills, except on the 

 south side which is separated from a tamarack swamp by a 

 low ridge. The lake has a clear sandy bottom and its beach is 

 narrow on the east and north sides. Here the waves are cutting 

 and the shore presents about the same conditions as are found 

 in the lakes above described. The south and west sides, how- 

 ever, furnish two Physa habitats which are of interest. ' The 

 first of these is found on the south side behind an ice-rampart, 

 Fig. 8. This rampart was formed by the pushing or crowding 

 of the ice on the shore during the winter.^ It extends nearly 

 three-fourths of the distance across the end of the lake and is 

 about 10 to 15 feet in width. It opens to the east and has two 

 gaps in it through which the water from the lake passes freely 

 at all times. During a storm the waves break over the full 

 length of the rampart and the waters flow back into the lake 

 through the gaps and at the open end. 



The protected waters behind this rampart furnish a good 

 habitat for Physa gyrina hildrethiana. The snails are found in 

 the shallow waters behind the protecting bar. Neither the 

 snails nor the water plants are evenly distributed in these waters. 

 Few plants or snails are found living exposed to the waves that 

 break through the gaps in the rampart, undoubtedly the wave 

 action is too severe. The snails are found in greatest abundance 

 on the shallow shore on both sides of the gaps. This was espe- 



» cf. Buckley, Ice Ramparts (1900). 



