250 OUR COMMON BRITISH FOSSILS. 



of the Isle of Man Sangidnoliies is locally very 

 plentiful. 



The Yoredale shales, and also the true Lower 

 Coal Measures, often contain an abundance of fossil 

 marine mollusca. Of these Posidonomya and Aviculo- 

 pecten papyracens are most numerous. The former 

 is a small, thin bivalve ; the latter of much larger 

 size. In the black shales which crop out in the 

 gorges and valleys near Hebden Bridge and Tod- 



Fig. 234. — Aviculo-pecten j>apy7'aceus Fig. 235. — Euom^halus pentangiilata 

 (Carboniferous formation). (Carboniferous limestone). 



morden, Aviculo-pecten is converted into iron-pyrites, 

 and the black shales look as if they had been gilded 

 all over with pictures of this fossil. In the shales 

 of the coal-seams in the Lower Measures near 

 Oldham, and also in a similar situation in strata of 

 about the same age at Halifax, Avictilo-pecten is un- 

 compressed, and usually converted into carbonate of 

 iron ore. This fossil is quite as abundant in the 

 shales of this age in Ireland as in England. At the 



