7o Arenig Slates. 



large scale, rest unconformably on the underlying rocks 

 of North Wales. 



In Cumberland the Arenig slates form the moun- 

 tains of Skiddaw and Saddleback, and from the borders 

 of the Old Red Sandstone, a few miles further east, they 

 stretch right across the country westward to Egremont 

 and northward to Sunderland, south of which town, near 

 Cockermouth, they are directly overlaid by the Carbon- 

 iferous Limestone. In that country they have usually 

 been called the Skiddaw slates. In Scotland the 

 Durness strata belong to the same rocks. 



In Britain the fossils that belong to this part of the 

 Silurian series are not very numerous, taken as a whole, 

 though some groups are remarkably abundant. As far 

 as observation has gone, Hydrozoa of the sub-class 

 Graptolitidse first appear in these strata, including some 

 20 genera and 48 species. The greatest number of 

 species belong to the genus Didymograptus, of which 

 20 species have been named, after which come Tetra- 

 graptus, Diplograptus, Dichograptus, and Dendro- 

 graptus. 



Eighteen genera and 47 species of Trilobites occur 

 in the same rocks, including Agnostus (A. hirundo, 

 &c.) ; Asaphus (A. Homfrayi, &c.) ; Ogygia (0. Sel- 

 wynii, &c.) ; Trinucleus (T. Ramsayi, &c.) ; Calymene 

 (C. parvifrons, &c.), and many others. Of Brachio- 

 poda there are 7 genera and 18 species including three 

 Lingulas, Lingulella Davisii and L. lepis, 7 species 

 of Orthis, including 0. calligramma and 0. lenticu- 

 laris ; 2 species of Obolella ; 2 of Discina, and others. 

 Of Lamellibranchiata there are only 5 genera and 8 

 species known, including Modiolopsis trapeziceformis, 

 Palcearca socialis, and P. amygdalus, Ctenodonta 

 elongata, &c., and Redonia Anglica. Pteropoda of 



