218 PEOCEEDIKGS OP THE GEOLOGICAL SOCIETY. [Feb. 21, 



at points so far removed from one another over the earth's surface, 

 that we can never suppose them to have been connected by a single 

 ocean the conditions of which were so uniform as to have allowed 

 the spread of the same animals over its entire extent. 



If we were thoroughly acquainted with the range of any given 

 fossil vertically, and, at the same time, had all the necessary details 

 as to what may be called its geographical distribution, we should 

 then be able to lay down pretty accurately the lines along which it 

 must have migrated when the conditions of its original area became 

 unsuitable for its further existence therein. It is also possible 

 that, in receding from its original area, we might find the species 

 becoming modified in difi^erent directions, and ultimately giving rise 

 to forms which we in our ignorance regard as well-marked varieties 

 or, perhaps, as distinct species. In order, however, to arrive with 

 any precision at such a conclusion, it would be necessary that we 

 should know both the vertical and the lateral range of the fossil, 

 and especially that we should know which of its several areas of 

 occurrence is its original one. The vertical range of a fossil, or 

 group of fossils, we may in many cases hope ultimately to learn 

 with accuracy; and we may approximately determine its lateral 

 range with suificient exactness for practical purposes. It is, how- 

 ever, in most cases difficult or impossible to point to the origmal 

 area in which a given fossil species appeared, and from which it 

 spread, when we have several areas to choose from. 



In the present state of our knowledge, any attempt to trace the 

 migrations of any given species or groups of fossils must be more 

 or less provisional and tentative. The present communication 

 is an endeavour to arrange, as far as may be, such facts relating 

 to the distribution of the Graptolites as enable us to sketch out 

 the migrations effected by these singular organisms, from their 

 rise in the Upper Cambrian period to their final disappearance at 

 the close of the Upper Silurian period. In carrying out this en- 

 deavour, the genera Dictyonema, Dendrograpsus, Callograpsus, and 

 Ptilograpsus will be left out of account, partly because they are only 

 doubtfully referable to the Graptolitidce, and partly because, in any 

 case, their habit of existence must have been very different from that 

 of the typiqal Graptolites, 



A. Skiddaw Aeea, — Leaving the above genera out of considera-- 

 tion, the earliest Graptolites of Britain are found occupying an area 

 in the north of England in which was deposited the dense mass of 

 muddy sediments known as the Skiddaw Slates. I need not enter 

 here into the age of these deposits beyond expressing my belief that 

 they wiU ultimately be shown to belong to the Upper Cambrian 

 period, and that they are certainly older than the Lower Llandeilo 

 rocks of Wales. Though making their appearance here for the first 

 time in Britain, the Graptolites are represented in the Skiddaw area 

 by numerous genera and species, the greater number of which are 

 peculiar. That the area was no small one is certain; and it can be 

 shown to have extended at least as far as Lower Canada. This is 

 proved by the large number of genera and species which are not only 



