154 LIMNJi:A PEREGRA. 



warm water is discharged, and which water during the day is con- 

 stantly rising in temperature, the mollusc crawls out of the water 

 when it reaches 120 degrees Fahrenheit. It has then stood as 

 much of the heat as it is able. It, however, returns to the water 

 again during the night, when the temperature is lower. 



This species varies very much ; in fact, there is no particular 

 in which there is not great variation. The shell may be very thin, 

 as in var. thermalis, or very thick, as in var. lutea ; the spire may 



^lJ^ 



Fig. 3. — var. therinalis. Fig. 4. — var. burnetii. Fig. 5. — var. viarginata. 



Fig. 6. — var. labiosa. Fig. 7. — var. obiusa. Fig. 8. — var. obloiiga. 



be produced, as in var. elongaHssitna, or even intorted, as in var. 

 burnetii; it may be dark brown or pure white, the lip may be 

 simple, as in the type, or it may be thickened, as in var. margiti- 

 ahi, or reflected, as in var. labiosa; the front margin of the shell 

 may be inflated and rounded, as in var. obtusa, or it may be com- 

 pressed and flattened, as in var. oblonga. A good deal of ingenuity 

 has been exercised by our ablest naturalists to account for the 

 divergence between the long form, which is generally accepted as 

 the type, and the globular or ovate forms, it being generally 

 accepted that the long, narrow forms inhabit streams where less 

 resistance is offered to the running water than would be the case 

 by the globular forms ; hence the latter inhabit ponds where there 

 is no motion in the water. Most of the Continental conchologists 

 regard var. ovata as a distinct species ; but Hazay looks upon 



