632 J. HOPEXSTSOX axe c. lapwoeth on the geaptolites of 



genus Dictyonema (Dictyograptus) is also mentioned as occurring 

 at Whitesand Bay, where it had been discovered by Mr. Hicks in 

 the lowest beds of the series. 



In 1872 the discovery in the Arenig rocks of Ramsey Island and 

 TVhitesand Bay of about twenty species of Graptolites, identical 

 with, or nearly allied to, those of the Quebec Group of Canada, and 

 related, but not so nearly, to those of the Skiddaw series, proved that 

 we had in these rocks a nearer representative of the Quebec Group 

 than was before known to be present in Britain* ; and in the fol- 

 lowing year, the discovery of a higher series of Graptolitiferous rocks 

 in Ramsey Island, with species all of which seemed to be identical 

 with those of the Skiddaw Slates, showed that the Skiddaw series 

 was of more recent age than the lowest portion of the Arenig rocks 

 of St. David's f. 



This summer (1874) several new species of Graptolites have been 

 found by Messrs. Hicks and Hopkinson in the Arenig rocks of TThite- 

 sand Bay ; and a few species have also been found in beds underlying 

 the Lower Llandeilo rocks of AbereiddyBay at a quarry near Llanvirn. 

 This assemblage of species is just such as occurs in the highest beds 

 of the Skiddaw Slates and Quebec Group, proving that we have here, 

 as in the higher Graptolitiferous rocks of Ramsey Island, beds of 

 Upper Arenig age. 



In the Lower Llandeilo series of Abereiddy Bay, again, several 

 species of Graptolites have been found, most of them being new ; and 

 a few species have also been collected in the higher Llandeilo rocks 

 exposed in Abereiddy Bay ; but here there is much more work to be 

 done before we can form any idea of the number of species they 

 may contain, and of the number of distinct zones into which their 

 species will enable us to divide them. 



Classification. 



The Graptolites to be described in the present paper include the 

 whole of the species collected within the last three years on the 

 occasions referred to, of which specimens sufficiently perfect for 

 determination or description have been obtained. 



The classification and nomenclature of the containing rock-groups 

 and their subdivisions is that adopted by Mr. Hicks in his Memoir 

 already cited on the Arenig and Llandeilo rocks of Pembrokeshire. 

 The arrangement of the fossils themselves will be found to differ in 

 several material respects from that generally current amongst palaeon- 

 tologists. The order in which they will be described is indicated in 

 the following synoptical table, in which the genera already known as 

 occurring in the Arenig and Llandeilo rocks of the neighbourhood of 

 St. David's are printed in italics. 



* Brit. Assoc. Eeport for 1872, Sections, p. 107. Proc. Liverpool Geol. Soc. 

 1872-73, p. 36. 



t Brit. Assoc. Eep. 1873. Sections, p. 82. Proc. Liverpool G. S. 1873-74, 

 p. 47. 



