Ch. XXVIL] specific agkeement of fossils. 445 



as MM. de Verneuil, D. Sharp, Prof. Hall, and others, who have entered 

 upon this comparison, admit that there is a marked general correspond- 

 ence in the succession of fossil forms, and even species, as we trace the 

 organic remains downwards from the highest to the lowest beds ; but it is 

 impossible to parallel each minor subdivision. In regard to the three fol- 

 lowing points there is little difference of opinion. 



1st. That the Niagara Limestone, No. 7, over which the river of that 

 name is precipitated at the great cataract, together with its underlying 

 shales, corresponds to the Wenlock limestone and shale of England. 

 Among the species common to this formation in America and Europe are 

 Calymene, Blumenbachii, Homalonotus delphinocephalus (fig. 587), with 

 several other trilobites ; Rhynchonella Wilsoni, and R. cuneata ; Orthis 

 elegantula, Pentamerus galeatus, with many more brachiopods ; Ortho- 

 ceras annulatum, among the cephalopodous shells ; and Favosites goth- 

 landica, with other large corals. 



2d. That the Clinton Group, No. 8, containing Pentamerus oblongus 

 and P. Icevis, and related more nearly by its fossil species with the beds 

 above than with those below, is the equivalent of the Middle Silurian as 

 above defined, p. 437. 



3d. That the Hudson Eiver Group, No. 12, and the Trenton Lime- 

 stone, No. 14, agree paleontologically with the Llandeilo flags, containing 

 in common with them several species of trilobites, such as Asaphus (Iso- 

 telus) gigas, Trinucleus concentricus (fig. 598, p. 441) ; and various 

 shells, such as Orthis striatula, Orthis biforata (or 0. lynx), 0. porcata 

 ( 0. occidentalis of Hall), Bellerophon bilobatus, &c* 



Mr. D. Sharpe, in his report on the mollusca collected by me from 

 these strata in North America,f has concluded that the number of species 

 common to the Silurian rocks on both sides of the Atlantic is between 30 

 and 40 per cent. ; a result which, although no doubt liable to future 

 modification, when a larger comparison shall have been made, proves 

 nevertheless that many of the species had a wide geographical range. 

 It seems that comparatively few of- the gasteropods and lamellibranchiate 

 bivalves of North America can be identified specifically with European 

 fossils, while no less than two-fifths of the brachiopoda, of which my col- 

 lection chiefly consisted, are the same. In explanation of these facts, it is 

 suggested that most of the recent brachiopoda (especially the orthidiform 

 ones) are inhabitants of deep water, and that they may have had a wider 

 geographical range than shells living near shore. The predominance of 

 bivalve mollusca of this peculiar class has caused the Silurian period to be 

 sometimes styled " the age of brachiopods." 



The calcareous beds, Nos. 15, 16, 17, and 18, below the Trenton Lime- 

 stone, have been considered by M. de Verneuil as Lower Silurian, because 

 they contain certain species, such as Asaphus (Isotelus) gigas, Illcenus 

 crassicauda, and Orthoceras bilineatum, in common with the overlying 

 Trenton Limestone.^ But, according to Professor Hall, the Illcenus was 



* See Murchison's Siluria, p. 414. f Quart. Geol. Journ. vol. W. 



% Soc. Geol. France, Bulletin, vol. iv. p. 651, 1847. 



