Some years ago Mr. D. M. S. Watson and I collected, from the Carboniferous Limestone of Clitheroe, Lancashire, a considerable series of Echinoids. One slab of the limestone (measuring 16 × 12 cm.) is covered by a great number of disjointed plates and other skeletal fragments of a large Perischodomus, probably P. bliserialis, M'Coy. Dr. R. T. Jackson refers to the specimen (Phylogeny of the Echini, p. 406), and, as the jaw fragments show features not hitherto described for the genus, it seems advisable to give a brief account of it. The preservation of a compass is a point of special interest, as these delicate structures are known in very few cases in the fossil state.