The recent discovery in the Woodwardian Museum of an undescribed species of Isopod from the Upper Greensand of Cambridge affords an opportunity for the revision of the entire list of that class of fossils. The total number of species which have hitherto been described as occurring in a fossil state is inconsiderable, —probably scarcely thirty—including both foreign and British. To what extent this small number expresses the variety of specific form of this tribe of Crustaceans, which actually existed during the period of deposition of the several rocks in which their remains occur, it is impossible to determine, as doubtless by far the greater proportion of the individuals perished by reason of the delicacy of their tissues—the larger and thick-shelled species only having been preserved—the small, thin-shelled kinds not admitting of recognizable “fossilization.” Specimens of Jurassic and Cretaceous Isopods are very rare both as to variety and individual number, and it may be inferred that this rarity of occurrence results from the more or less turbulent conditions under which these marine deposits were formed. The Tertiary estuarine and freshwater species, buried under more tranquil conditions, are much better preserved, and occur in some localities in innumerable abundance—Sphœroma, Archœoniscus, etc.