¹ These, with many additional forms obtained by us from the Arenig rocks of St. David's, were afterwards described by Messrs. Hopkinson and Lapworth in the Quart. Journ. Geol. Soc. vol. xxxi. p. 632 (1875), and in the discussion of that paper Mr. Hopkinson said “that the dendroid forms are only known to occur in abundance in Britain in the Arenig rocks of St. David's, and that there are intermediate forms connecting British and American species which occur in rocks of more ancient date”. He further remarked that he “did not consider the dendroid forms valuable for determining zones, species very nearly allied to those of the Arenig rocks being met with even in the Lower Ludlow rocks of Shropshire; but the Rhabdophora occur only in small zones, and wherever they are found they seem to hold an equivalent position. They are consequently valuable for stratigraphical purposes”. in the same paper they give a Table “in which every special (obtained in the vicinity of St. David's) is referred to its exact position in the vertical series”, and in page 639 they say: “Perhaps the most patent result of these researches is the circumstance that they clearly demonstrate that the Hydroida of these ancient rocks, so long shunned or misinterpreted by the systematist, are rapidly emerging from the obscurity which has enveloped them, and will perhaps soon stand side by side with the better understood Brachiopoda and Crustacea, as unerring exponents of the true geological age of the most widely separated rocks in which they are found”.