6 JEAN DAWSON 



habitats that vary widely, as a glance at the preceding diagram, 

 Fig. I, will show. Brook X follows the road which runs north 

 and south on the east side of the swamp and represents 

 the optimum for that region, Fig. 2. It rises on the border 

 of the moraine, seen in the distance, and flows north to where 

 it joins the large ditch draining the lake. The bottom of the 

 ditch is covered with soft, decaying debris, over which algae 

 and Char a grow in thick masses. Above these water plants, 

 flows water from two to three inches in depth, \vith some current 

 in the spring and during heavy rains, but rather sluggishly at 

 other times. The snails live upon this mass of plants which is 

 growing over a bed of organic debris. This habitat was very 

 well stocked with Physa gyrina in the spring months, but during 

 the dry fall it was examined again, and the snails had greatly 

 decreased in number. Many were found dead. 



Another brook-ditch arises on the north edge of the swamp 

 and flows toward the south to empty into the lake ditch just 

 described. This presents conditions almost like the optimum 

 except that there is very little water over the plants and the 

 current is always feeble. Physa gyrina is not abundant in 

 this habitat except near the mouth where it empties into the 

 swiftly flowing lake ditch. (Fig. 2.) 



The remaining ditches of this swamp contain a less number 

 of snails on the average than does the optimum and present 

 somewhat different features which are as follows: There is 

 usually less than one-half the number of plants and the current 

 is perhaps twice as strong as that found in the optimum habitat. 

 Instead of decaying vegetation there is a clean substratum of 

 peat. The temperature of the water varies in different parts 

 of the brook-ditch, due perhaps to springs bubbling up through 

 the peat ; at least water is seen to bubble from the ground and 

 is two or three degrees colder than the surrounding water. 

 The snails gather in greater numbers in the warmer pools; 

 and there is practically no shade. 



2. Creeks. No attempt is made to describe as a whole the 

 creeks examined, and, in general, account is taken only of such 

 portions of their waters as contain Physa. Comparison is also 

 made with other regions of very similar appearance, but which 

 lack Physa in order to learn if possible what disturbing element 

 or elements have modified the normal environment. Physa 



