It is nearly forty years since Walcott published the well-known memoir which first gave palæontologists definite information regarding the appendages of Trilobites. This work was based on a laborious investigation of Calymene senaria and some other species by means of thin sections. Since then much knowledge has been gained, and, in particular, Beecher's researches on Triarthrus have provided us with a new conception of the Trilobite limb which, in some respects, is not readily to be reconciled with Walcott's earlier results. Now, at length, in this finely illustrated monograph, the veteran student of the Trilobita brings together the results of his own work and that of other investigators and reviews the whole in the light of his unrivalled experience.