indebted to the researches of Wilfred Mark Webb, Esq., F.L.S., F.Z.S., and to those of Miss Johnson and her sister. Some of the species, it is true, are so uniformly distributed over the whole neighbourhood that their existence in this county could not have been reasonably disputed. But the examples which my very obliging friends have sent from definite localities dispense with any necessity of depending on speculative inference, and, as will be presently seen, the results are not confined to those customary forms the occurrence of which could have been predicted. Here, as in all other counties, the Amphipoda are represented by Gammarus pulex (Linn.). It has the advantage of being everywhere obtainable from ponds and brooks. But it is of small size, so that for the proper understanding of its separate details some optical enlargement is almost essential. Otherwise it is a convenient object of study for the beginner, not only by reason of its great abundance, but also because in many respects it seems to offer a standard of comparison, a simple pattern from which the very numerous genera and species of Amphipoda diverge and radiate. Its insignificant size might readily put an observer off the thought of comparing it with a crayfish, but the organization is essentially the same. Here however the shield or carapace, instead of being produced backward as far as the pleon, stops short over the first maxillipeds. Hence there is left uncovered the middle body, consisting of seven segments, which as a rule are all articulated. To each of these is attached a pair of appendages. The first two pairs, which are generally used for grasping, have been called gnathopods, and these are homologous with the second and third maxillipeds of the crayfish. Maxillipeds and gnathopods alike mean jaw-feet, the words being intended to teach that the appendages in question are foot-like in form or in origin, but concerned with the food in point of function. All but the first of the seven pairs of legs may carry branchial vesicles, comparable with the more complicated gills which are hidden beneath the cheeks of the carapace in the higher crustaceans.
Along with G. pulex at Eton and at Iver Mr. Webb has taken the isopod Asellus aquaticus (Linn.). The two species are almost constant companions and perhaps equally abundant, but at some times and some places one or the other may be found to predominate in numbers. The Isopoda can scarcely be considered so united a group as the Amphipoda, and if any genus could be selected as a central representative it would scarcely be Asellus. Nevertheless, as the only aquatic isopod of our inland counties the species of it above mentioned deserves attention. It is as curious as it is common. Like other genuine isopods it agrees with the amphipods in having sessile eyes and seven uncovered articulated segments of the middle body, and differs from them in having its breathing organs in the pleon instead of attached to gnathopods and walking legs. But an extreme ventral flattening gives it a highly peculiar appearance. By this shape it is enabled to adhere very closely to the leaves and stems of the water weeds about which it climbs. Those who are at the pains to compare its appendages pair by pair with those of
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