Pleurophjllidia loveni (probably the Diphyllidia lineata of Hancock), Genia (Acteonia) coclcsi, and Gumanotus beaumonti, or giving some details of the dentition and anatomy of these and a few other species.As is natural in a supplementary part, the majority of the animals figured are rare, but this rarity may be the result of accidental circumstances as much as of real scarcity.Limapontia, Genia, and Alderia escape notice owing to their small size. Doris testudinaria is commonly confounded with D. tuberculata, but is not uncommon on our southern coasts and probably elsewhere. Pleurophyllidia loveni appears to live in sand at moderate depths and can as a rule be obtained only by dredging. Doris maculata and Grimora papillata are perhaps southern forms which only occasionally reach our shores. Gimianotus on the other hand appears to be a northern form which is, nevertheless, established at Plymouth. It is probable that it occurs on other parts of the British coast but is local in its distribution.The drawings are accompanied by various notes in Hancock's writing, mostly disjointed and not prepared for publication. They are cited from time to time in the pages which follow, but the text of the present part is not otherwise based on them.To the synopsis of the British fauna and the descriptions of new species I have added chapters on the distribution, nomenclature, bionomics, classification, and affinities of the Nudibranchiata. It is remarkable how little of Alder and Hancock's work has proved to be incorrect, and later researches have added less than might be expected to their accounts of such species as they were able to investigate thoroughly.But in the Mono-graph (which does not, however, represent all their work) they neglect several families, and since their time numerous new forms have been discovered which have materially affected our ideas as to the morphology and classification of the group.I have endeavoured to indicate the character and extent of these additions to our knowledge, at least as far as they concern the Nudibranchiata of the northern Atlantic.I must thank the Council of the Hancock Museum at Newcastle-on-Tyne for their courtesy in lending me all the drawings, notes, specimens, and microscopic preparations left by Alder and Hancock. 1 The preparations consist chiefly of radulee and have been used for some of the figures of teeth (Grimora, Hero, Tritonia alba, and Doris testudinaria) included in the present plates.My best thanks are also due to Pi'ofessor W". C. Mcintosh for the loan of a collection