The subjoined Note summarises an investigation into the skeleton of the pectoral fin of a common Selachian—Acanthias vulgaris—with the object of determining whether, and if so, to what extent it shows any vestige of the archipterygial type of structure. To this end careful examination was made of five hundred random embryos, and the result demonstrated the general occurrence of vestigial post-axial rays at the tip of the fin. The suggestion that such rays—stated by Braus to occur in a specimen of the embryo of Centrophorus—are to be regarded as a mere casual variation without evolutionary significance is thus shown to be devoid of weight, and the pectoral fin skeleton of Selachians established definitely in line with that of Pleuracanthus and Ceratodus.