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WALTER KEEPING OjS" THE 



out by Mr. Lapworth. And here the reading is decisive : our Cardi- 

 ganshire series belongs not to the Glenkiln (Llandeilo) or the Hartfell 

 (Bala) periods, when the Didymograpti and Diplograpti nourished, but 

 to that part of the newer or Silurian era when the Monograptidse 

 were predominant, and in their greatest development, namely the 

 Llandovery period (Birkhill Shales). 



The more detailed affinities of our fauna will appear by a study 

 of the last five columns of the " Table of Distribution." Every one 

 of our species of true Graptolitidse (excluding the Cladophora) occurs 

 in the Upper Birkhill beds of Scotland. Eight species are common 

 to the Lower Birkhill Shales, and nine (as also two species of Cla- 

 dophora) to the overlying Gala and Tarannon groups. 



Of the fossils of the two principal Graptolitic localities, Cwm 

 Symlog and Morben, Mr. Lapworth writes that they "lie within the 

 same general Graptolitic zone." ..." Of the nine species recognized 

 in the slates of Morben, five (namely Rastrites maximus, Carr., 

 Monograptus spinigerus, Mch., M. distans, Portl., M. Hisingeri, 

 Carr., and Dvplograpsus Hughesii, ISi ich.) make their first appearanco 

 in the Upper Birkhill Shales of S. Scotland. Rastrites maocimus is- 

 confined to the highest zone of that formation, in the Moffat area, 

 and, together with its common associate, Dipl. Hughesii, seems to 

 have become extinct before the deposition of the overlying beds of 

 the Gala group. 



" The second fossil locality (Cwm Symlog) is even more strikingly 

 marked by its Upper Birkhill Graptolites. Sixteen forms have been 

 identified from this spot. Of these, seven species (viz. Monographs 

 crenidaris, Lapw., M. intermedins, Carr., M. Cling ani, Carr., M. 

 runcinatus, Lapw., M. Hisingeri, Carr., Dipl. Hughesii, Mch., and 

 D. sinuatus, Nich.) are forms which are known for the first time in 

 the Upper Birkhill of S. Scotland, and its equivalents in Girvan, 

 Ireland, and the north of England. M. intermedins, M. runcinatus, 

 and M. crenidaris are peculiar to the Upper Birkhill, as also is M.. 

 Clinqani, which is confined to a small seam in the very centre of the 

 Birkhill beds." 



Of the nine species of true Graptolites common to our Mid-Wales 

 rocks and the Gala and Tarannon, not one is a special Tarannon 

 form. Monograptus turricidatus, however, is most frequent in those 

 beds ; and the two species of Cladophora Dictyonema venustum, 

 Lapw., and Rhizograptus ramosus, Lapw., are only known, else- 

 where, in the Gala group (Scotland). 



Erom the fossil evidence, therefore, there can be no hesitation in 

 referring our Mid-Wales rocks to the same age as the Upper Birk- 

 hill of Scotland; and beyond the occurrence of the three species 

 just mentioned in the rocks of the Devil's Bridge, there is nothing 

 that conveys the slightest hint that any of our strata are newer than 

 Upper Birkhill. 



Lake District. — Eifteen of the Mid- Wales Graptolites are known 

 from the Coniston or Graptolitic Mudstones of the Lake District ; so 

 that, in Mr. Lap worth's words, " the general facies of this Mid- 

 Wales fauna is distinctly that of the Coniston Mudstones." These 



