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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