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A HISTORY OF ESSEX of structure which assists them in breathing air. The colour of P. ratzeburgii is described as grey, with a row of six white spots along each side of the back. For the Amphipoda which, like the Isopoda, are fourteen-footed sessile-eyed malacostracans, there are no available printed records, but I can scarcely omit to mention the occurrence of Jassa pulchella, Leach, Corophium crassicorne, Bruzelius and Caprella linearis (Linn.), since speci- mens kindly given me by Dr. Sorby indicate that these well-known species are as abundant in Essex waters as in many other localities. Also on the oyster-testing excursion above mentioned, I was myself able to obtain specimens of Hyperia galba (Montagu) from its accustomed habitat in the common jelly-fish Aurelia aurita. The results thus brought together, however scanty in themselves, are at least suggestive that Essex, as might be expected from the position and character of its coast-line, will be found to provide very abundant and attractive resources for students of marine carcinology. For those interested in the crustacean fauna of inland waters, there is no need to rely on conjecture or presumption. In regard to freshwater Entomostraca the labours of Mr. D. J. Scourfield have placed this county in the front rank. Twelve years ago there was not a record traceable for any single species of this group with definite locality assured to it. At present, although the subtle manoeuvres, the diminutive size, or the absolute rarity of some species may have left them to be gleaned by future researches, already Mr. Scourfield has been able to enumerate more species from this county than have yet been recorded from any other, having found in Essex more than a hundred species out of a total of less than two hundred known from the British Isles at large. To make any account intelligible of this great number of species, an outline. must be given of their classification. There are three principal companies, called Branchiopoda, Ostrac6da, Copepoda. The Ostracoda, or shelly group, have their unsegmented bodies boxed up in a pair of valves, as if they were little molluscs. The Copepoda, by name and nature oar-footed, have the body segmented and not enclosed in a bivalved shell. These however, when parasitic, often become subject to strange vagaries of structure, which set definition at defiance. The Branchiopoda are so called from the branchial or respiratory character proper to their limbs. They include three subdivisions, Phyll6poda, Clad6cera, Branchiiira, among which difference of appearance is often quite as prominent as likeness. The Phyllopoda are again divided into sets far from closely resembling one another, since one set has a carapace and another set has none, while the third has the body almost enclosed in a pair of valves. Cbirocephalus diaphanus^ already noticed, belongs to those that have no valves nor carapace, yet being of all our freshwater Entomostraca though unadorned adorned the most. The Branchiura are a very small and rather perplexing group, represented in our islands chiefly by the long-known Argulus foliac e us (Linn.), a disk-like parasite on various fishes and tadpoles, with its 210