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 plcbeia. 



geologist finds them creeping over shells and encrinite 

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



Figs. 173, 174. — Fenestella plebeia((Zzx\i(i\\\i^xQM% 

 limestone, Halkyn, etc.). 



Fig. ijs.—Glaucon07ne 

 Jlexicarinata. 



The limestone shales of the Silurian and Carboniferous 

 formation contain them in great plenty, as is indicated 



