374 FRESH WATER MOLLUSCAS THAT 



in the land's surface. Generally they are fed by springs, 

 and there is an outlet for the superfluous water, after the 

 cavity is filled up by boggy, or turfy matter, and the quag- 

 mire overgrown by shrubs, trees and smaller plants. 

 The learned world is greatly indebted to Silvanus Mil- 

 ler, Esq. for an admirable topographical description of 

 this district. The shells of these inhabitants of 

 the fresh water had been collected as long ago as 

 1803, by Dr. Samuel Akerly, and placed with the other 

 specimens which constitute the body of eonchology com- 

 pleted by his industry. Great praise, notwithstanding, is 

 due to Thomas Say, Esq. for the skill and science he has 

 displayed in the classification of these and other mollus- 

 cas of fresh water ; outstripping all his cotemporaries, 

 and clearing the path for his followers. 



The shells found in the pools are small and delicate, 

 and are of the following species : 



1. A carinated planorbis* 



2. A plain planorbis. 



3. A lymniaea. 



4. A spirorbis. 



5. A tellina. 



Their remains, after death, dissolve to marl, or a white 

 ealcarious carbonate like chalk, easily effervescing with 

 vinegar, and exceedingly valuable as a manure. Until 

 the fertilizing properties of gypsum were discovered, 



these marl-pits were mines of wealth to the proprietors. 



t 



But, as it has been believed by some naturalists, that 

 those shells belonged to marine animals, I state it as a 

 fact, that D'Jurco Knevels has seen all the species just 



