It is now just fifty years since Burmeister wrote that the “was convinced” that the reasons he afforded would be “deemed sufficiently conlclusive to satisfy the unprejudiced reader” that “the tribolites were a peculiar family of the crustacea, nearly allied to the existing phyllopoda, approaching the latter family most nearly in its genus Branchipus, and forming a link connecting the phyllopoda with the pŒ cilopoda.” Burmeister's reasoning has not, however, been generally considered satisfactory, and his claim that the trilobites are related to the phyllopoda, though recognized as possible, appears somwhat to have waned before the claim put forward by others that they are primitive isopds. But this latter relationship had already been shown by Burmeister to be highly improbable, and this judgement is fully endorsed and further enforced by Gerstaecker, whose monumental review of the crustacea in Broon's ‘Klassen und Ordnungen des Thierreichs’ gives special weight to his opinion. In the absence of any certain knowledge as to the character and arrangement of the limbs, Gerstaecker, while recognizing trilobites as crustacea, declines to adopt any special relationship: that is, he is evidently not convinced by Burmeister's arguments were, in themselves, far from conclusive, even when correct as far as they went. Since the appearance of the 5th volume of Bronn's ’Klassen und Ornungen’ 1879, however, further facts have come to light which completely justify the conclusions of Burmeister, so far, that is, as to the trilibites having been primitive phyllopods.