334 . Mr. C. Lapworth on the Geological 



Oedovician or Lower Silurian System. — Notwithstand- 

 ing the great additions made of late years to our knowledge of 

 the fossils of the rich graptolitiferous zones of the Llandovery 

 and Wenlock formations, Salter's well-known generalization that 

 Murchison's Lower Silurian system is most prolific in Grapto- 

 lites remains practically undisturbed. The Arenig formation 

 also, though it can no longer be defined as the birthplace of 

 the family, must still be acknowledged as its metropolis ; for 

 if we have respect to the abundant extra-British forms de- 

 tected in this formation, it must be conceded that " nowhere 

 else are there so many species and such complex forms " *. 

 This may, it is true, be owing to the fact that not only in 

 Britain, but also in Europe and America the rocks of Arenig 

 age are essentially dark carbonaceous shales or schists more 

 or less graptolitiferous throughout; while the succeeding forma- 

 tions are marked by a much greater development of sandstones 

 and limestones, from which, as a rule, Graptolites are absent. 

 Nevertheless, even where black shales crowded with Grapto- 

 lites do occur in these more recent formations, we never find 

 so great a variety of types as upon any single horizon in the 

 Arenig ; and it may be that the Tlhabdophora (like the 

 Cladophora) had their culmination at or near the commence- 

 ment of the Arenig, the loss, especially in genera, being very 

 rapid as we pass upwards into the Llandeilo and Bala. 



Arenig Formation. 



The only Welsh rocks belonging to this formation as yet 

 carefully searched for Graptolites are the Arenig rocks of the 

 neighbourhood of St. David's, familiar togeologists through 

 the extended researches of Dr. H. Hicks. The three sub- 

 groups into which the strata of this age are there divisible all 

 yield Bhabdophora in some abundance. The forms recog- 

 nized by Mr. Hopkinson and myself include f ' — 



I. Lower Arenig. 



Didymograptus exteusus, Halt. Phyllograptus Stella, Hopk. 



pennatulus, Hall. Trigonograptus truncatus, Lapw. 



sparsus, Hopk. ensiformis, Hall. 



II. Middle Arenig. 



Didymograptus patulus, Hall. Tetragraptus serra, Brongn. 



Tetragraptus Halli, Hoj?k. quadribraclriatus, Hall. 



? Hicksi, Hopk. Clematograptus implicatus, Hopk. 



* Quart. .Tourn. Geol. Soc. vol.xix. pp. 119 et seq. 



t Hopk. & Lapw. Quart. Journ. Geol. Soc. vol. xxxi. p. 634. 



