&4 THE STORY OF THE EARTH. 



indent I shells known are found 



in the Tremadoc rocks of Wales. They can be 



illeled at the present day. Modiolopsis 

 is pi nothing but Modiola, the horse mus 



under another name ; and Glyptarca, Palaarca, and 

 forms of the living genus Area, 



in which the shell has not developed the habit of 



ring in depth along its hinge, as in most of its 



living representatives. Pteropods are well repre- 

 id: the univalve Gasteropods are represented 

 by the extinct genus Bellerophotiy which appears to 

 be a symmetrical shell abundant in the primary 

 rocks, probably allied to the living Pleurotomaria. 

 The group of many-chambered shells named C'eph- 

 poda is represented by allies of the Nautilus, 

 of them the Straight horn OtthoceraSy and an- 

 yrtoceraSy the curved horn. In the lower 

 rocks the oldest known starfish is 

 found, in a species of the genus Palaasterina j 

 and the oldest known Crinoid or Stone-lily, in a 

 of the genus Dendrocrinus* 

 -real Axenig series rests conformably on 

 emadOC slates. It forms Cader Idris, the 

 Kest niog Mountains, Aran and Arenig. It in- 

 due 'eat group Of roofing slates worked in 



i >f l est ini( >g, and an immense quan- 

 tity of volcanic ash. The total thickness of the 

 - and lavas seen in ( lader [< 



IS 1 5000 and 6000 feet. The throat^ of 



ent volcan< is w hich 1 ontributed so larj 

 to form the Arenig n m ks in \< >rth Wales, •> 



: 1 . and Aran Mowddwy by the 



mbrian period was an epoch of \ . 

 u tion, The products of the vol- 

 Skiddam slates of the Lake 



