WHILE collating the plates in a recently acquired copy of the Revision of the Echini (A. Agassiz, 1872–4), I noticed with surprise a representation of complex ambulacral structure in “Echinobrissus” recens (pl. xiv a, figs. 3–4). A style of plate-grouping essentially similar to that in Echinonëus (tab. cit., figs. 7–8) is portrayed as affecting all the extra-petaloid parts of the five areas, and encroaching slightly on the distal parts of the petals. No reference to this unusual condition occurs in the text. As the proofs of my recent paper on ambulacral structures (Phil. Trans., B, vol. 209) were in my hands at the time, I was immediately impressed by the remarkable resemblance between “E.” recens and the Cretaceous Trematopygus—a correspondence by no means restricted to ambulacral plating, but involving all coronal features save the orientation of the peristome.

Mr. W. A. Richardson's paper in the December, 1919, number of this Magazine has furnished a very useful summary of the problem of the origin of flint, with some very interesting suggestions; but it cannot be allowed to pass altogether unchallenged.

WITH the exception of the Catalogue of Fossil Cephalopoda by Foord in the British Museum (Natural History) and the same author's monograph Carboniferous Cephalopoda of Ireland, the bibliography of this family as represented in Carboniferous time is scattered through many volumes and publications. Not one of the publications makes any attempt to define the precise horizon in the Carboniferous Series at which any one of the species occurs, or indicates its persistence in geological time.

WITH regard to the laws governing the transport, sorting, and deposition of sediments, much has still to be learned, but one fact emerges clearly from the mass of present data. During the process of transport by water the load undergoes selective treatment, or, in other words, is sorted. The late Dr. G. K. Gilbert, in the course of his important work on the transportation of débris by running water, carried out experiments with mixtures of several grades of detrital material and found that before the slope of the stream had been definitely established (i.e. before the attainment of uniformity of conditions), and especially when low velocities were used, the current tended to sort the débris, building deposits with the coarser part and continuing to transport the finer material. This fact has a special bearing on the study of natural sediments. The state of equilibrium, or of perfect uniformity of conditions, is seldom, if ever, completely realized in nature, and even when approximately reached is not, as a rule, of long duration.