I have lately been urged to revise the Cretaceous species of Entomostraca, enumerated in my Monograph, published by the Palæontographical Society in 1849. This I ought to have done ten years ago, after the publication of memoirs on the Cretaceous Entomostraca of Germany, the Netherlands, etc., by Reuss, Bosquet, and others, wherein some of my determinations of species are corrected from actual comparison of specimens. An improved classification was attempted in my Monogr. Tert. Entom., 1856; but a total revision is still more necessary now, because the late researches by Sars and G. S. Brady, in the structure and character of the recent Entomostraca, have multiplied and elucidated species and genera of these little bivalved Crustacea to an amazing extent. So clearly has Mr. Brady's “Monograph of the recent British Ostracoda” (Linnean Soc. Trans., 1868), in particular, defined the several groups, that we may in very many cases form a safe opinion of alliances from the shape, hinge, and other characters of the carapace-valve, at least far more satisfactorily than when in 1849 (Monogr. Cret. Entom.), and in 1856 (Monogr. Tert. Entom.), I laboured to institute some definite distinctions for fossil Entomostraca, founded on these characters.