86 THE STORY OF THE EARTH. 



me much more important and still survive; 



while the genus Leptaena, which now first ap- 



ves on till the lower part of the second- 



oh i>\ time. 'The common gen us Mytilus % 



edible mussel, is first found with the clost 



tbrian period. Cephalopods become more 



numerous and varied, and Cystidians are repre- 



ted by a number of genera. The appearance 

 and abundance of Graptolites, and the increase of 



Trilobites in number of genera and species, are 

 chief changes which occur in the life of the 

 ( lambrian period of time. 



The Silurian. 



The Silurian rocks extend unconformably 

 Over the Cambrian Strata. Between the Long- 

 mvnd and W'enlock Edge, they cover up the 

 whole series of the Cambrian strata, resting upon 



their upturned and denuded edges. But when 

 . are traced into North Wales in Denbigh- 

 shire, the evidence Of Silurian unconformity is 

 less marked. 



The Silurian rocks typically include the May- 

 hill Sandstone, the W'enlock rocks and the Ludlow 

 rocks. 'The May Hill series, s<> named from May 



Hill in Gloucestershire, consists chiefly of sand- 

 stones and Conglomerates, yellowish and brown 



with oxide of iron, about icco feet thick, covered 

 the Wenlock group, which in the south is 

 ted of shales and limestones, and in Denbigh- 

 shire chieflj i kn< w n as the I Denbigh 



, w huh overlie the Tar.innon shales. Ai 



• be i ;diow beds, \\ hich also in« 



d by the \ nes, 



i ks is i apped by the 



