FOSSIL SEA-MATS. 



215 



They abound in the marine deposits of all the Palaeo- 

 zoic rocks, from the Lower Silurian upwards. The 



Fig. 172. Fenestella. pleleia. 



geologist finds them creeping over shells and encrinite 

 stems, and they are often spread out in a fan shape. 



Figs. 173, 174. Fenestella filebeia (Carboniferous 

 limestone, Halkyn, etc.). 



Fig. i-j^.Glauconome 

 Jlexicarinata. 



The limestone shales of the Silurian and Carboniferous 

 formation contain them in great plenty, as is indicated 



