1872.] HICKS TREMADOC ROCKS. 43 



Tremadoc rocks of North Wales for some years, states that he has 

 no difficulty in recognizing in these heds at St. David's the equiva- 

 lents of the lower portion of the series in his district ; and as evi- 

 dence of their parallelism he mentions the occurrence of Niobe, 

 Dikelocephalus (Neseuretus?), and an Orthis similar to one of the 

 species at St. David's, but smaller, at the base of that series. It 

 is also an interesting fact that the genus Niobe in both districts is 

 only found in these lower beds ; and, again, the Dikelocephalus men- 

 tioned is truly a Neseuretus, a genus which, I think, will be proved 

 to belong to this horizon, not only in this coiintry, but also in Canada 

 and the United States. Prof. Hall, in 1863, in his 'Preliminary 

 Notice of the Fauna of the Potsdam Sandstone of the Upper Mis- 

 sissippi Valley,' mentions that the typical species of the genus 

 Dikelocephalus do not appear until we get to the later stages of the 

 formation. This is well borne out also by researches in this country ; 

 for the forms doubtfully figured and described by Mr. Salter (in the 

 Appendix to Prof. Ramsay's memoir) as Dikelocephalus from the 

 Upper Lingula-flags and from the Lower Tremadoc rocks, are, in my 

 opinion, species of Neseuretus, and the only true Dikelocephalus 

 found in Wales is the Dikelocephalus furca from the Upper Tremadoc 

 rocks of North Wales. 



The Upper Tremadoc rocks are, in Mr. Homfray's opinion, repre- 

 sented at St. David's by the so-called Arenig rocks, which are known 

 to contain several Upper Tremadoc fossils in addition to the rich 

 fauna of Graptolites discovered during our recent researches, and 

 which have been recognized by Mr. Hopkinson as belonging to the 

 age of the Quebec group of Canada. According to Mr. Homfray 

 some of these graptolites also have been discovered by Mr. Ash in the 

 Upper Tremadoc rocks of North Wales. 



In the present paper, however, I have only included the rocks 

 below the so-called Arenig group, as known at St. David's, in the 

 Tremadoc group, believing that the two formations are sufficiently 

 distinct lithologically and palseontologically to be separately con- 

 sidered. 



If Mr. Homfray's supposition, however, is proved to be correct 

 (and it is supported by the fact that the Tremadoc series as hitherto 

 considered is much greater in thickness in North Wales than in South 

 Wales, and the Arenig series much less), I think it will necessitate 

 the doing away with the name Upper Tremadoc, and also a change in 

 the boundary line, hitherto placed above the Upper Tremadoc, and 

 which has been looked upon as of considerable importance in stra- 

 tigraphical classification, to its base, as the whole Arenig series is 

 much more intimately allied to the overlying Llandeilo slates than 

 it is to the underlying Tremadoc rocks as exhibited at St. David's. 



and of a deep sea in the other, at the same time. It is not at all unlikely also 

 that the lower portion of the Tremadoc series at St. David's was deposited con- 

 temporaneously with the black beds of the Lingula-flags of North Wales, as in 

 both cases they are the first indications of a change taking place in the sea-bottom 

 after the long period of the shallow sea in which so many thousand feet of 

 Lingula-flags and sandstones were deposited. 



