1872.] NICHOLSOK MIGRATIONS OF THE GEAPTOLITES. 231 



their place may be taken, as palseontological guides, by tbe genera 

 Didymograpsus and Dicranograpsns, neither of which has as yet been 

 detected in any Upper Silurian deposit. 



Discussion, 



Mr. Etheridge commented on the importance of Dr. Mcholson's 

 paper, and on the difficulties attending the sttxdy of the Graptohtidse. 

 The migration of these organisms appeared to him to be very diffi- 

 cult to establish, especially in connexion with their extension both 

 eastwards and westwards. 



Mr. Hughes believed that if we could discover the original of any 

 species, we should see a small variety appearing among a number of 

 forms not very different from it, and from which it had been de- 

 rived ; but when the variety had prevailed, so as to be the dominant 

 form, we were far on in the history of the species ; that it was a 

 great assumption to fix upon any bed we now know as representing 

 the original source of any group ; that we know too little about the 

 chronological order of the geological divisions referred to to reason 

 with any safety on the migration of Graptolites from one area to 

 another — that the term Loiver LlandeUo, for instance, was very un- 

 satisfactory as used in the paper ; there Avas nothing lower than the 

 Llandeilo Mags at Llandeilo ; and where older beds occurred in 

 Scotland and elsewhere, it Avas not at all clear that the equivalent of 

 the Llandeilo Flags was present at all. He differed also altogether 

 from the author as to the position of the Dufton Shales, and criti- 

 cised the views of the axithor as to the range of some species. He 

 thought that M. Barrande's theory of colonies was borne out by 

 the study of the Graptolites, but that we had not sufficient data to 

 speculate as to the areas in which they made their first appearance, 

 or the order of their geographical distribution. 



Prof. Duncan observed that, at the present time, there was among 

 other forms quite as great a range of species as that of the Grap- 

 tolites pointed out by the author. Having looked through all the 

 drawings of Graptolites that he could meet with, he had found none 

 whatever that were accurate ; and he had moreover never in any 

 specimens discovered such cups or calices between the serrations as 

 were always attributed to these organisms. Prom all he had seen 

 he was led to the conclusion that the projections on the GraptoUtes 

 bore the same relation to the central stem as those of some of the 

 Actinozoa. These latter also, like the Graptolites, seemed to prefer 

 a muddy sea. Professor Duncan also suggested that the Graptolites 

 were really the remains of the filiform poly piferous parts of floating 

 Hydrozoa. 



Prof. MopvEis regarded the paper as mainly suggestive. It was 

 on all hands agreed that there were in Britain two principal zones 

 in which graptoHtic hfe was most abundant j and the same held 

 good in America. Both these seemed to be homotaxially related. 

 M. Barrande had long since pointed out the probable migration of 

 many of the Bohemian species from the British area; and there 



