ANXIVEESAET ADDEESS OE THE PRESIDENT, 



145 



been during the formation of the Arenig, Llandeilo, Caradoc, and 

 Lower Llandovery deposits, as shown in the following table : — 



Arenig 17 genera and 24 species. 





18 



55 



44 



35 





21 



55 



38 



35 



Lower Llandovery of South Wales 



5 



33 



17 



53 



Lower Llandovery of Westmoreland 



6 



5J 



25 



33 



Lower Llandovery of Scotland 



/ 





52 



35 





4 





12 







9 



55 



23 



33 



Ludlow 



3 



)? 



8 



55 



I mention these facts and results 



here 



rather 



than under the 



Ludlow section, as the group of the GraptolitidaB are conspicuous 

 here, and they are of little value in classification above the Llando- 

 very rocks. It may be remembered that in 1872* Prof. H. A. 

 Xicholson published, in the Journal of the Geological Society, a 

 paper of great originality and highly suggestive of the mode of mi- 

 gration and distribution of the Graptolites in time and space, or 

 the vertical and lateral range of this group of fossils. In this paper 

 Prof. Xicholson endeavours to show the peopling of one area as 

 derived from another and from points widely separated, showing 

 that under such circumstances they can never be truly contempo- 

 raneous. " In the present state of our knowledge, however, it must 

 be more or less provisional and tentative " to attempt to trace the 

 migrations of any given set of species or groups of fossils : but pro- 

 bably the time will arrive when definite groups nnder given conditions 

 may receive elucidation for lateral range, especially when the original 

 area of occurrence and dispersion is known. Dr. Nicholson argues, 

 and justly, that " if we were thoroughly acquainted with the range 

 of any given fossil or species vertically, and were conversant with 

 all the details of its geographical distribution, we should then be 

 able to lay down with some degree of accuracy the lines along which 

 it must have migrated when the condition of its original area be- 

 came unsuitable for its further existence therein." Prof. Xicholson 

 endeavours to show in his paper certain facts relating to the distri- 

 bution of the Graptolites, so as to enable us to sketch out the mi- 

 grations of these and other organisms from their first appearance in 

 time ; in the case of the Phabdophora from the Arenig to their final 

 disappearance at the close of the Ludlow deposits. Pive areas are 

 selected to illustrate the distribution of the Graptolites : — 1st. The 

 Skiddaw group and its Arenig fauna, as being the earliest in which 

 the Graptolites appeared, and probably the first area, for we have no 

 reason to believe that the Canadian or Quebec deposits much preceded 

 in time our own Cumberland series ; the large number of genera 

 and species not only common to the Skiddaw Slates of the north of 

 England and the Quebec group, but exclusively confined to them, 

 conclusively and distinctly prove that the area was no small one. 



* Quart. Journ. Geol. Soc. vol. xxviii. pp. 217-232 (1872). " Migrations of 

 Graptolites," by H. A. Nicholson, M.D., D.Sc, &c. 



