220 PROCEBDINGS OP THE GEOLOGICAL SOCIETY. [Feb. 21, 



this horizon iu Britain, and which died out at the close of the Skid- 

 daw period. These genera, with one exception, likewise did not 

 survive the close of the Quebec period in North America ; and there 

 is therefore reason to conclude that the termination of this period 

 was effected in a manner which must have resulted in a large ex- 

 tinction of forms, only a few species, as we shall see, appearing to 

 have accomplished a successful migration. In fact, out of a total of 

 thirty-one species known in the Skiddaw Slates only seven seem to 

 have survived to be preserved in later deposits ; and the extinction 

 seems to have been all but universal in the Canadian area. 



Besides the above-mentioned genera, there appear iu the Skiddaw- 

 area for the first time the genera Dldymograpsus, Cllmaeograpsiis, 

 and Dlplograpsus {PUurograpsus is only doubtfully represented, and 

 may be left unconsidered). Of the eight species of Dldymograpsus 

 all but two are peculiar in Britain to this horizon ; but D. geminus 

 and i). patidus survive into the Lower Llandeilo. The migration of 

 these two characteristic species appears to have taken place both 

 southwards and eastwards from the Skiddaw area ; for we find them 

 both in the Lower Llandeilo rocks of Wales, and in the Lower Grap- 

 tolitic Schists of Aher, in Scandinavia. A third Dldymograpsus, 

 viz. D. serralulus,^Q]l, ajDpears to have survived the Skiddaw period 

 and to have migrated in a westerly direction, since it is wanting in 

 the later deposits of Britain but turns up after a prolonged lapse 

 of time in rocks of Caradoc age (Utica Slate) in North America. 

 This is the more noticeable, as the species is wanting in all the de- 

 posits older than the Caradoc in the United States. 



Of the CUmacograpsl one very characteristic species is peculiar to 

 the Skiddaw Slates, as are three species of Dlplograpsus. There re- 

 main Climacograpsus tereilusculiis, His., O. bicornls, Hall, Dlplo- 

 grapsus pristis, His., and D. miuironatus, Hall, which survive into 

 younger deposits. These species seem to have migrated from the 

 area of the north of England at the close of the period of the Skid- 

 daw Slates ; and the course of the migration appears to have been in 

 all cases the same. All these species, namely, appear in the Upper 

 Llandeilo Eocks of "Wales, the south of Scotland, and Ireland ; so 

 that the dispersal of these long-lived forms was general. It would 

 not appear, however, that they extended their range very far beyond 

 the British area in the earlier portion of the Lower Silurian period. 

 Some of them reached the Scandinavian area, along with the two 

 Dldymograpsl above alluded to ; but all these forms are wanting in 

 the Silurian basin of Bohemia in the earlier phases of Barrande's 

 " second fauna." 



B. Llandeilo Akeas or Wales and Scotland. — There are good 

 grounds for the belief that the north of England was in the con- 

 dition of dry land during the earlier portion of the Llandeilo period. 

 At any rate, whether the close of the Skiddaw period was signalized 

 by the upheaval of the Lake-district (as I believe), or whether the 

 seas of the succeeding period were rendered unfit for life by intense 

 igneous activity, it is certain that no deposits containing Graptolites 

 are superimposed upon the Skiddaw Slates until we reach the later 



