THE Pennatulacea of the Siboga expedition-the richest collection of sea-pens made by a single expedition-comprise about 550 specimens, which Prof. Hickson has referred to seventeen genera and forty-five species (seventeen new). Having at his disposal this, wealth of material, and other specimens in his own collection and in that of the University of Manchester, Prof. Hickson has taken the opportunity of making a thorough survey and revision of the order. He regards the order as consisting of only a few well-marked generic groups, and considers that new generic names should not be introduced without very strong reason. No new generic name is proposed in this memoir, and several recently described genera have been merged in older ones. Prof. Hickson carefully defines the descriptive terms employed in his memoir in the hope that henceforward there may be a greater measure of uniformity in the terminology; certainly he has set a high standard of precision in the systematic descriptions.