ch. xxyii.] specific agreement of fossils. 571 



such as MM. de Verneuil, D. Sharpe, Prof. Hall, E. Billings, and 

 others, who have entered upon this comparison, admit that there is 

 a marked general correspondence in the succession of fossil forms, and 

 even species, as we trace the organic remains downwards from the 

 highest to the lowest beds; hut it is impossible to parallel each minor 

 subdivision. In regard to the three following 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 Ccdymene Blumenbachii, Homalonotus delphinocephalus (fig. 639, 

 p. 559), with several other trilobites ; Rhynchonella Wilsoni, and 

 Retzia cuneata ; Orthis elegantula, Pentamerus galeatus, with many 

 more brachiopods ; Orthoceras annulatum, among the cephalopodous 

 shells ; and Favosites gothlandica, 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 denned, p. 560. 



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

 stone, No. 14, agree pafeontologically with the Caradoc or Bala group, 

 containing in common with them several species of trilobites, such as 

 Asophus (Isotelus) gigas, Trinucleus concentricus (fig. 647, p. 563) ; 

 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 gastero- 

 pods 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 collection 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 

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



* See Murclrison's Siluria, p. 414. 

 f Quart. Geol. Joum., vol. iv. 



