1872.] 



NICHOLSON- MIGEATIONS OF THE GEAPTOUTES. 



^19 



common to the Skiddaw Slates of tlie north of England and the 

 Quebec group of Canada, but which are exclusively confined to these 

 formations. The differences between the GraptoHtic faunas of the 

 Skiddaw and Quebec regions are not greater than can be well set 

 down to imperfect observation or to the inevitable eff'ect of difference 

 of station, and certainly do not necessitate our supposing that the 

 one region was peopled by migration from the other. Nor is the 

 distance between the north of England and Canada so great as to 

 compel our acceptance of this hypothesis ; whilst the two regions 

 are nearly in the same latitude, and are not separated by any im- 

 passable barriers. The case is different, however, with Australia, 

 in which some of the peculiar genera of the Skiddaw and Quebec 

 groups have been discovered. Here we are compelled to assume 

 that we have a case of migration, though we have at present no 

 data whereby to decide whether the course of the migration was 

 from Canada to Australia (as is most probable), or vice versa. 



The following Table exhibits the species of Graptolites known to 

 occur in the Skiddaw Slates, those which are common to the Quebec 

 group being marked with an asterisk : — 



*Dichograpsus ocfcobrachiatus, Hall. ^ 



gracilis, Nich. 



annulatus, Nich. 



- — — multiplex, Nich. 



reticulatus, Nich. 



*Loganograpsus Logani, Hall. 

 *Tetragrapsus bryonoides, Hall. 

 *___ Headi, Hull. 



* crucifer, Hall. 



* quadribracliiatus, Hall. 



*PliylIograpsus angustifolius, Hall. 



* typus, Hall. 



* Anna, Hall. 



Trigonograpsus lanceolatus, Nich. 



Didymograpsus afRnis, Nich. 

 * bifidus, Hall. 



• fasciculatus, Nich. 



^ nitidus, Hall. 



V-fractus, Salt. 



*Climacograpsus antennarius, Hall. 



Diplogi'apsiis arraatus, Nich. 



— Hopkinsoni, Nich. 



*^ • pristiniformis, Hall. 



Pleurograpsus (?) vagans, Nich. 

 *Didymograpsus patidus, Hall. 



geminus, His. 



Cliraacograpsus teretiusculus. His. \ 



— — bicornis, Hall \ g^p^j^g j^to the Caradoc period, either 



DiplograpSUS priStlS, yi<5. Y i„ T>,.,-fain n,. i,-, AmPViVn 



niucronatus, Hall. 



Didymograpsus serratulns, Hall. 



> Peculiar to the Skiddaw group. 



I Survive into the Lower Llandeilo. 



From the above Table it will be seen that the Skiddaw area was 

 characterized by no less than five genera (viz. Bicliograpsus, Tetra- 

 (jrapsus, PhyJlograpsus, Loganograpsus, and Trigonograpsus, repre- 

 sented in all by fourteen species) which are exclusively confined to 



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