88 Tin: story OF THE EARTH, 



ill the Carboniferous limestone, which is locally 

 formed of many different organisms. There are 



twenty-five genera and seventy-six species of 



.is in the Wenlock rocks alone. And twenty 



■ era of crinoids appear, the group being 



represented by sixty-eight species in the Wenlock 



limestone. This limestone is frequently thin- 

 bedded, and alternates with shale, often green, as 

 is well seen in the dome-shaped exposure in the 

 Wren's Nest, near Dudley, where it is covered 



Coal measures without any intervening rocks. 

 The thin beds of limestone both in the Wenlock 

 and Ludlow beds, thin out, first becoming nodu- 

 lar and concretionary, and then disappearing 

 altogether. 



In Wales and the adjacent parts of England 

 the Wenlock rocks have been very little meta- 

 morphosed, and are in this respect in marked 

 contrast to the cleaved slates of the Cambrian 



id. They have probably a wide distribution 



tinder the secondary strata, ami were met with 



Chalk and Gault at Ware, in a deep 



water. The characteristic fossils 

 With the exception of the remarkable 

 crinoids, corals, and brachiopods, there is noth- 

 ing very im] e in the character i^\ the Silu- 

 rian faima. The remarkable crus Vpte- 



tus % and Hemiaspis first appear in the 



i ks. 

 I ,,- oldest fishes in Britain are met with in 



fewer than 

 \-, ,ng these is the Buckler-headed 



in the I I I dlOW, while m the 



found CephalaspiSt afterwards 



known in the old gether with 



and other gen< 



