THE UPPEK SILTJKIAN OF NORTH AMERICA. 57 



place quotes from Sir A. Eamsay. The Ludlow rocks are considered 

 by these writers as the correlates of the lower part only of the 

 Lower Helderberg (Water Lime) of New York, leaving the upper 

 portion of the Lower Helderberg also without any English equiva- 

 lent. In both these respects the views of Professor James Hall 

 coincide with those above stated. In the 'Palaeontology of New York ' 

 (vol. iii. p. 34) he writes : — 



" Since the presence of Eurypterus is regarded as marking the 

 uppermost strata of the Silurian system of Great Britain, our Lower 

 Helderberg group constitutes a series of strata not recognized and 

 probably not existing in the British Isles." 



Professor Hall then gives the following table illustrating his views 

 on the correlation of these beds : — 



New York. Great Britain. 



Lower Helderberg. 



Water Lime. Lesmahago Beds. 



Onondaga Salt Grroup. 

 Niagara Group. Wenlock Limestone. 



The inference is reasonable that these Pennsylvanian Pteraspids 

 are older than the Scaphaspis ludensis of England, by the time re- 

 quired for the deposition of 200 feet of strata. 



To avoid returning to this subject, I wiU here add that the English 

 Wenlock group is placed on the horizon of the American Niagara 

 on evidence which has never, so far as I know, been disputed. Sir 

 R. Murchison wrote in 1859 (Siluria, p. 460) : — 



"The Niagara shales in all respects resemble the weU-known 

 Wenlock shale of Britain ; whilst the chief or central mass of the 

 Upper Silurian rocks in North America is that called the Niagara 

 limestone, which unquestionably represents the Wenlock and Dud- 

 ley limestones of England as well as of Gothland in the Baltic. 

 These rocks appear to contain a greater number of fossils identical 

 with those of Europe, than do the Lower Silurian strata of the same 

 districts ; '' among them are the following : — 



List of Fossils common to the American Niagara and the English 

 Wenlock Limestone. 



Calymene Blumenbachii, Bro?ig7i., Orthis hybrida, Sby. 



now 0. niagarensis, Hall. 

 Homalonotus delphinoeephalus, Orthoceras annulatum, Sbt/. 



Green. 

 Bumastus barriensis, Murch., now Eucalyptocrinus decorus, FJdll. 



Illaenus loxus, Hall. 

 Rhynchonella cuneata, Hall, now ? Bellerophon dilatatus, Sby. 



Rhynchotreta cuneata, var. 



americana. 

 Ehynchonella Wilsoni, Sby. Favosites gothlandicus, Lam. 



? Pentamerus galeatus, Balm. Favosites alveolaris, Be Blainville. 



Orthis elegantula, Balm. Halysites eatenulatus, Linn. 



Excluding two species from the list, for the presence of which in 

 the Niagara I can find no evidence, and considering the close resem- 

 blance of the three that now pass under other names, we have twelve 

 species common to the two groups. Their equivalence admits of no 

 question. 



