After reading the review of Prof. Wyville Thomson's “Depths of the Sea,” in the May Number of the Geol. Mag., in which, a figure and short description are given of a new living Echinoderm, which Prof. Thomson has named Calveria hystrix, one of whose peculiar features lies in its possessing overlapping plates, it may be interesting to readers of the Magazine to know that besides the other species with overlapping plates noticed in the above-mentioned review, there were Echinoderms having their plates arranged on the same plan as long ago as the Lower Carboniferous Limestone period. Those to which I specially refer belong to the genus Archœocidaris.