222- PEOCEEDINGS OF THE GEOLOGICAL SOCIEXy. [Feb. 21, 



from the Skiddaw area, some of the species remaining unchanged. The 

 genus Dldymograpsus is represented here for the last time, so far as 

 England and Scotland are concerned ; whilst all the species are pecu.- 

 liar, and none of them seem to have been actual immigrants from the 

 Skiddaw area. Some of them, however, as we shall see, migrated west- 

 wards, and reappear in strata of Caradoc age in North America. It 

 should be noticed also that there is here a remarkable difference 

 between the Scotch and Welsh areas, though both seem to have been 

 stocked from the north of England. Four species, namely, of Didymo- 

 grapsus appear in the Scotch area; and these are both widely diffused 

 and abundantly represented by individuals. On the other hand, but 

 one DidymograiDSUs (viz. D. MurcMsoyii) occurs in the "Welsh Upper 

 Llandeilo area ; and this is extremely local and limited in its distri- 

 bution. Climacograpsus, again, and Bijjlograpsus, whilst derived 

 from the older Skiddaw area, have also representatives of their own ; 

 and both pass on into newer deposits. It may be noticed, however, 

 in this place, that the only species of the Llandeilo GraptoUtes of 

 Wales which would seem to have escaped extinction is CUmaco- 

 grapsus teretiuscidus, His., which is also found in the Lower Llan- 

 dovery — unless, indeed, the Diplograpsus pet'scidptus of Carruthers, 

 which also occurs in the Llandovery rocks, may be regarded as a 

 modified descendant of D. pristis, His. With one or two doubtful 

 exceptions, the great and varied genus Oraptolites appears to have 

 its commencement in the Upper Llandeilo area of the south of Scot- 

 land, where it is most abundantly represented as regards both specific 

 types and individuals. It passes on, also, into much younger deposits. 



Lastly, the genus Dicranograpsus would seem to have been intro- 

 duced into the Uj)per Llandeilo area of the south of Scotland by 

 migration from the Welsh area, where it is represented in the Lower 

 Llandeilo, This genus dies out here, so far as England and Scotland 

 are concerned ; but, in company with many other forms, it would 

 appear to have migrated westwards at the close of the Upper Llandeilo 

 period, being found in rocks of Caradoc age in the United States. 



The subjoined Table exhibits the genera and species of Graptolites 

 of the Upper Llandeilo area of the south of Scotland, arranged in 

 groups, according as they have been derived from preexisting areas, 

 are peculiar (so far as known) to the Upper Llandeilo period, or pass 

 onwards into higher deposits : — 



Up)per Llandeilo Graptolites of Scotland. 

 A. Genera. 



Didymograpsus 1 



Climacograpsus \ Deriyed from the Skiddaw area. 



Diplograpsus J 



Uicranograpsus Derived from the Lower Llandeilo area of Wales. 



Pleuroo-raDsus | I'robably peculiar to the Upper Llandeilo, but possibly 



^ ^ I commencing in the Skiddaw period. 



Cajnograpsus "^ 

 , ( = Helicograp- | Commence in the Upper Llandeilo, and pass onwards 

 sus) y into higher deposits, either in Britain or in other 



Rastrites j areas. 



.: iRetiolites } - 



