CAMBRIAN AND ORDOVICIAN ROCKS. S3 



of r >und two genera oi fossils named 



///(/, which may be regarded as 



havi g - vived through all subsequent ages oi 



time to the present day without under- 



ny notable change, although the surviving 



I is named Lingula instead ^i Lingulella, The 



low- 3 in which the , the Caerfai group, 



led by the Solva group, in which genera 



the extinct order of Trilobites appear in profu- 



llrre occurs the oldest known sponge named 



LS met with of the ex- 



1 named T/uwi, which sur- 



d in the ancient sea> f or a time. This 



assembl life, the s yet known in 



the earth's hist ts of types which are in 



•nbryonic. It distinctly points to a 



line of similar ancestors which has yet to be dis- 



W'ith the i <>t the overlying 



nevian I ng divisions ol 



the Lingula flags and Tremadoc slates, a fauna 



of 1 f fossils becomes known in which 



CARBONIFTROUA 



TRIAS 



i through North Wales from the Cambrian slates 

 of C . district to the Cheshire I 



: the shells, species of the genera of Brachi- 



opoda named Lingula and Orthis, and Obolclla \ 



up into the overlying Arenig :s. With the 



\h nevian beds the most ancient Echinoderm 

 appears. It is a far-off relative of the existing 

 stone-lines and sea- ggs. It belongs to the ex- 

 tinct group of Cystidea, and is named Protocystites. 



