1872.] NICHOLSOJT — MIGRATIOXS OP THE GEAPTOLITES. 221 



l^ortiou of the Bala period, at which time an immigration of Grap- 

 tolites must have taken place from neighbouring seas. This being 

 the case, the further course of the Graptolitic tree must be followed 

 in other areas in which Lower and Upper Llandeilo deposits are 

 developed. We must, then, change our ground, so far as Britain is 

 concerned, from the north of England to Wales and Scotland. 



In the Welsh area alone are Lower Llandeilo rocks developed ; 

 and we find in these Dichjmograpsus r/eminus, His., D. patulus, Hall, 

 CUmacograpsus teretmscidus, His., C. hicornis, Hall., Diplogmpsm 

 mucronatus, Hall, and probably B. pristis, His., all of which reached 

 the AVelsh area by migration from the Skiddaw area. With these, 

 however, are Graptolites sar/ittarius, His., and Dicranograjpsus ra- 

 mosus, Hall, the former being only very doubtfully present. The 

 genus Dkranograpsus is represented here for the first time ; and, so 

 far as we yet know, this may be regarded as the centre of dispersal 

 of this afterwards widely distributed genus. 



In the Upper Llandeilo area in Wales are found, of the species in 

 the Lower Llandeilo, only Climacograjpsus teretiusculus, C. hicornis, 

 Diplograpsiis prist is, and Dicranograpsus ramosus. All the other 

 Lower Llandeilo forms appear to have died out in the Welsh area at 

 this period. Didymograpsus patidus and D. geminus, especialh', 

 may be regarded as characteristic of the Lower Llandeilo horizon, 

 since they never appear in any higher deposits in any other area. 

 The only new form which appeared in Wales in this period is Didy- 

 mograpsus Murcliisoni, Beck. This beautiful species never seems to 

 have succeeded in migrating out of the Welsh area ; but there are 

 reasons for believing that it is merely a very slightly modified de- 

 scendant of the Didymograpsus hijidus of Hall, which abounded in 

 the Skiddaw area, whence it probably migrated southwards. 



The species of the Skiddaw area Avhich migrated northwards into 

 Scotland are Climaeograpsus teretiuscidus, C. hicornis, Diplograpsus 

 pristis, and D. mucronatus; and all of these are represented in the Upper 

 Llandeilo rocks of Dumfriesshire. With these old species, however, 

 are now associated a large number of fresh forms, most of which are 

 unknown in the corresponding rocks in Wales. As to the genera, we 

 meet for the first time with liastrites, Ccenograpsus (=HeUcograpsus), 

 Retiolites, and most probably Pleurograpsus and Oraptolites. There 

 are also many new species of the old genera Climaeograpsus, Diplo- 

 qrapsus, and Didymograpsus. Of all these genera, liastrites, Cceno- 

 grapsus, and most probably Pleurograpsus appear to have had their 

 origin in the Dumfriesshire area, so far, at any rate, as can be judged 

 from the fact that they occur in no older deposits in any other region. 

 The two latter did not survive the close of the Upper Llandeilo period ; 

 but liastrites not only passes on into later deposits, but becomes one 

 of the most widely distributed genera of the GraptolitidiE. Retiolites 

 is represented in the Skiddaw and Quebec groups by the allied genus 

 Trigonograpsus ; but the typical forms of this genus appear here for 

 the first time. The genus survives, however, into far younger rocks, 

 and becomes very widely distributed. The genera Didymograpsus, 

 Climaeograpsus, and Diplograpsxis, as already indicated, were derived 



