IO2 Devonian Strata. 



identical with those found in the Lower Carboniferous 

 Limestone shales. 



There is in England a considerable diminution in 

 the number of Devonian fossils when compared with 

 those of the Silurian rocks. Thus about 1,500 species 

 of Silurian fossils are named, while of marine Devonian 

 we have under 400 species, and adding those of all 

 kinds in the freshwater strata of the Old Eed Sand- 

 stone, 535 species. Of corals, 11 of the genera only 

 are also Silurian. Of Echinodermata, there are 10 

 genera and 21 species, only 3 of the genera being also 

 Silurian; Crustacea, 13 genera, 35 species, 5 of the 

 genera being also Silurian, including those found both 

 in the Devonian rocks and the Old Red Sandstone. 

 In the latter no Trilobites occur, but only Crustacea 

 of the genera Eurypterus (6), Pterygotus (4) (fig. 26), 

 Stylonurus (7), while in the Devonian formations of 

 Devonshire we find 5 genera of Trilobites: Bronteus 

 (B. flabellifer) Cheirurus 2, Phacops 6 5 Homalonotus 

 2, and Harpes J, all being genera common in the 

 Silurian strata, though the species are distinct. Twelve 

 of the Devonian genera of Brachiopoda occur in 

 Silurian rocks, but of 96 Devonian species few pass 

 downwards, and these are doubtful. The most prevalent 

 genera of Brachiopoda are AtJiyris, Atrypa., Cyrtina, 

 Orthis, Rhynchonella, Spirifera, Streptorhynchus, and 

 Terebratula. Species of the genera Leptcena and Pen- 

 tamerus decline in numbers, while Orthis, Rhyn- 

 chonella, and Spirifera are much increased. Of 21 

 genera and 60 species of Lamellibranchiate molluscs, 

 the species are all, or almost all, distinct from those 

 of Siluria, while only 6 of the genera are the same. 

 The most prevalent forms are Aviculopecten (10), 

 Pterinea (9), Cucullcea (7), and Ctenodonta (7). 



