Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/279

 their place may be taken, as palaeontological guides, by the genera Didymograpsus and Dicranograpsus, neither of which has as yet been detected in any Upper Silurian deposit.

Discussion.

Mr. Etheridge commented on the importance of Dr. Nicholson's paper, and on the difficulties attending the study of the Graptolitidae. The migration of these organisms appeared to him to be very difficult 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 derived ; 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 Lower Llandeilo, for instance, was very unsatisfactory as used in the paper ; there was nothing lower than the Llandeilo Flags at Llandeilo ; and where older beds occurred in Scotland and elsewhere, it was 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 criticised the views of the author 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 Graptolites 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. From all he had seen he was led to the conclusion that the projections on the Graptolites 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 polypiferous parts of floating Hydrozoa.

Prof. Morris regarded the paper as mainly suggestive. It was on all hands agreed that there were in Britain two principal zones in which graptolitic life was most abundant; 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