In the Geological Magazine, Vol. VII., p. 260, PI. VII., Figs. 2–5, we published a description, by Prof. de Koninck, of a new British Cystidean, which the author named Placocystites Forbesianus. We called attention in a footnote (op. cit., p. 261) to the probability of Prof. de Koninck's Cystidean being the same as Mr. Billings's Ateleocystites Huxleyi; but as we had not seen the Canadian specimen upon which the genus was founded, we abstained from speaking very positively as to their identity. We, however, prepared a sketch of Placocystites, which we forwarded by Sir William Logan, who kindly undertook to submit it to Mr. Billings, in order to obtain his opinion thereon. We have been favoured with the subjoined letter, which has, however, been accidentally mislaid, and so for a time delayed in publication, for which we beg to apologize.