A HISTORY OF BERKSHIRE pairs are generally subchelate, that is to say they terminate in a sort of incomplete though sufficiently effective pincers. These appendages are equivalents of the last two pairs of mouth-organs in the crayfish and other decapods, while the remaining five pairs of legs tally with the ten feet from which the decapods derive their designation. Another important characteristic of the Amphipoda is found in the branchiae, which, instead of being greatly subdivided and concealed in branchial chambers on either side of the carapace, are commonly of simple structure and unenclosed. They are attached to the bases of some or all of the last six pairs of the thoracic limbs. Between the two species that have been already mentioned, Gammarus pulex and Niphargus aquilex^ the dis- tinction is fairly easy. The former is of a yellowish or greenish brown colour, with dark eyes, and the two branches of the terminal appendages only a little unequal in length ; the latter is white and pellucid, with the eyes imperceptible, and the terminal appendages distinguished by the great length of the outer branch and the rudimentary condition of the inner. The late Mr. Spence Bate described three other species of English 'well-shrimps,' two of them being additions to the genus Niphargus. One of these is found near Maidenhead. Specimens of it were procured for me from that locality by one of my former pupils, Mr. H. F. Cowper-Smith. Between this species, N. kochianus, and the nearly allied N. aquilex some points of distinction are very apparent from the figures given in the well known volumes by Bate and Westwood. If attention be directed to the pleon, that is, the part of the animal behind the legs, it will be seen that the large second and third segments have the postero-lateral margins broadly rounded in N. aquilex but acute- angled in N. kocbianus. The two front pairs of limbs, known as gnathopods, are shown with ' hands ' longer than broad in the latter species, but as broad as they are long in the former. Also the adjustment of the ' hand ' to the ' wrist ' differs in the two forms. 1 The figures referred to cannot perhaps be trusted for very minute accuracy, since the equality of size between the first and second gnathopods attributed to both species does not really belong to either. The first gnathopods certainly as a rule are in both species decidedly smaller than the second. Not improbably in the case of N. kocbianus, instead of the first and second, the second have been figured in duplicate. A more exact study of the species however has recently been made by Dr. Charles Chilton, M.D., D.Sc., and his paper in the Journal of the Linnean Society* on ' The Subterranean Amphipoda of the British Isles,' cannot be dispensed with by those who take an interest in this subject. He points out two additional distinctions which are important, although the student will scarcely be in a position to verify them without carefully dissecting his specimens. When the fourth pair of mouth-organs, known as the maxillipeds, are flattened out under the microscope, it will be perceived that the large spine-bordered plate of the third joint in N. kocbianus 1 Brit. Sess. Crust, pt. 7 (1862), i. 315, 323.
- Vol. xxviii. (1900), 140-61, pis. 16-18.
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