Summary.1The radulæ of certain Octopodidæ have multicuspid median teeth. In many species these teeth exhibit a serial arrangement in which the ectocones in successive sets of teeth occupy a progressively more external position.2In certain species this seriation is alternating, the series on the opposite sides of the main cusp not being coincident.3The distribution of this phenomenon, uniqne in the Mollusca, is described among the genera of the family; and some attempt is made to find an explanation for the serial arrangement, in the developmental history of the radul