TRANSACTIONS OF THE SECTIONS. 183 



alteration in their types of structure. The following summary of the results of the 

 researches of Professor James Hall, and other skilful palaeontologists, will show the 

 extent of this revolution in palaeozoic life. Unluckil)^ neither Hall nor any other 

 naturalist, has yet advanced to an exhaustive description and enumeration of the 

 American species above the Scalent and Niagara series, though it is possible to 

 glean valuable data from his essay on the Paleeozoic Deposits of the United States 

 and Europe, and from other sources whereby to make the comparison between the 

 Cambrian -Silurian break and the other later horizons of life discontinuity. 



The annexed Table presents in a summary form the palaeontological relations of 

 the American and European palaeozoic strata, indicating the numbers of the species 

 restricted to the several groups, and the numbers which are common to related ones. 



It is a striking fact, bearing directly on the present inquiry, that the proportion 

 of organic forms common to the lower and middle palaeozoic divisions, scarcely 

 exceeds zero. According to Professor Hall *, the number of species now examined 

 from the lower palseozoics of the United States surpasses 400, and those from the 

 Levant, Surgent, and Scalent series, or all but the uppermost Silurian formation, 

 are about 344. He also statesf, that already more than 200 species have been re- 

 cognized in the Premeridian or Lower Helderberg limestone, from which it would 

 appear that the Silurian or Upper Silurian of Great Britain have yielded about 550 

 forms. The two sets together have thus turned out about 950 species, or nearly the 

 number catalogued by Professor Morris as found in the corresponding formations 

 in the British Islands. Now it is a most instructive fact, that, out of these nearly 

 1000 lower and middle palaeozoic fossils, only three or four, if as man}', span the 

 great break which divides the two groups of rocks. This complete extinction of 

 the earlier or Cambrian races, is a circumstance so important in the comparative 

 palaeontology of the two continents, that it deserves to be dwelt on sufficiently to 

 show the precise extent of the evidence. Professor Hall, speaking of the Medina, 

 Clinton, and Niagara groups, states, " In these investigations, some new facts have 

 been brought to light, which all the previous examinations have not shown, the 

 discovery of several species of fossils heretofore known only in the lower rocks, [n 

 the western part of the State of New York, the lower beds of the Clinton group 

 have furnished very dilapidated specimens of Bellerophon Irilohafus with Delthyrus 

 Lynx, and one or two imperfect specimens of a Leptmna undistinguishable from L. 

 allernata. A few other fragments and imperfect specimens have also been found, 

 which appear to be forms belonging to Lower Silurian strata. These facts are ex- 

 tremely important and interesting, and I take the present occasion of recording them, 

 from the circumstance that all our investigations previously had only strengthened 

 the opinion that no fossils of the lower rocks had passed the Oneida conglomerate." 



When we compare this remarkable palfeontological break, amounting to certainly 

 99 per cent, of all the discovered organisms from the two sets of strata, with the 

 synchronous break, separating the Cambrian or older Silurian from the Silurian of 

 Great Britain, we find a marked diflFerence in the extent of the discontinuity in the 

 vital stream. Sir Roderick Murchison has shown in an Appendix to his work 

 ' Siluria,' in a Table of the vertical range of the older palaeozoic fossils, compiled 

 by Mr. Salter, that not less than 114 species are common to the lower and upper 

 groups. This number, assuming 880 as the species accessible for comparison, is 

 nearly 13 per cent, of the two entire faunas compared. It is obvious, therefore, 

 that the life-break, like the mechanical, was even more complete in the Appalachian 

 portion of the American paleozoic basin, than it was in the British part of the 

 European. A little more than one half of the 880 species enumerated by Professor 

 Morris, belong to the Upper Caradoc, Wenlock, and Ludlow formations, while, 

 according to Mr. Salter's list, 114 species, that is to say, about 22 percent., range 

 from the Llandeilo into these upper rocks. 



We reach a still clearer apprehension of the relative magnitudes of the American 

 and the British palaeontological breaks at the Matinal or Caradoc period, when we 

 regard for a moment the additional evidence afforded by comparing the proportionate 

 number of genera which pass the boundary in the two countries. According to 

 Professor Phillips's condensed enumeration framed from Professor Morris's Cata- 



* Palaeontology of New York, vol. ii. p. 319. 



f Foster and Whitney's ' Geology of Lake Superior,' 



