FOSSIL MOLLUSCA--BIVALVES AND UNIVALVES. 251 



pretty spa of Lisdoonvarna, county Clare, it crowds 

 them everywhere and the chemical decomposition 

 of the pyrites, these fossils were originally converted 

 into, has probably originated the sulphur springs. 



Fresh-water mollusca take the place of the marine 

 forms in the Upper Coal Measures. The most abun- 

 dant is the genus Anthracosiay or " fossil mussel." Its 

 dark-brown shells, often much flattened and crushed, 

 may be found in the greatest abundance on the shale- 

 heaps which accumulate near the coal-pits in Lan- 

 cashire, Cheshire, and Yorkshire. In the colleries near 

 Wigan several species may be obtained ; one kind, 

 A. robusta^ so named on account of its greater size, 

 is most abundant about the pits where the " Arlcy 

 mine" is worked for coal. These Anthracosia arc 

 sometimes so numerous that the surfaces of the 

 shale are completely covered thereby. Not unfrc- 

 quently they are converted into argillaceous iron ore, 

 and can then be picked out of the soft shale like nuts. 

 At times they form bands of ironstone, rich enough 

 to be worked and smelted, as at Carron, in Scotland, 

 where the celebrated " Blackband " ironstone is formed 

 of nothing but minerally altered mussel-shells. The 

 geological student can hardly go to a coal-pit in 

 Lancashire and Yorkshire, especially in the neigh- 

 bourhoods of Manchester and Barnsley, without finding 

 plenty of Anthracosia. 



The Permian rocks of England are nowhere par- 

 ticularly rich in fossils — except, perhaps, a few localities 



