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 A HISTORY OF BERKSHIRE front of the carapace. The carapace is the great cephalothoracic shield which covers the bases of the eye-stalks, antennae, mouth-organs and legs. The orbits are excavated on either side of the rostrum. The transversely sutured telson is at the opposite extremity to the rostrum, being the last of the twenty-one segments which may be assumed as constituents of the head, thorax and abdomen (or cephalon, perason and pleon) of a stalk-eyed malacostracan. The gills or branchiae are limited to the head and thorax, and are concealed under the carapace. They are called podobranchias when attached to the basal joint of an appendage, arthrobranchiae when attached to the membrane connecting this joint with the supporting segment, and pleurobranchia? when attached to the side of the segment itself. In Potamobius there are six pairs of the first sort, eleven of the second, and of the third one pair well developed and either two or three pairs that are rudimentary. The reckoning of twenty-one segments is spoken of as an assumption, because out of the first fourteen, which belong to the cephalothoracic division, all but the last are here so firmly soldered together that their original individuality, though not a matter of any reasonable doubt, is a matter of inference. In regard to the distinctive characters above arrayed, a beginner will very likely not find it especially easy to determine whether his specimen has three pairs of rudimentary pleurobranchias or only two, but the other differences are not difficult to observe. In the case of fresh specimens assistance may be derived from the colouring of the limbs, these being so much redder in one form than in the other that the noble crayfish has been distinguished as the ' red-clawed ' from the ' white-clawed ' stone crayfish. The value of this distinction is much enhanced by the evidence adduced that, notwithstanding the provision of ample oppor- tunity, the two forms do not interbreed. 1 That our English species is the stone crayfish seems to be placed beyond doubt, but in distinguishing that species as Astacus torrentium from the other and larger form which he calls A. nobilis, Huxley has chanced to be unlucky in all his names. From the more extended researches of Dr. Walter Faxon it appears that the German name Steinkrebs, with its Latin equivalent A. saxatilis, has covered two varieties or very nearly related species. Out of much confusion the scientific names torrentium (Schrank, 1803) and pallipes (Lereboullet, 1858) emerge as the earliest available names for the forms in question, pallipes belonging to the one found in England. On the other hand, the ancient name Jluviatilis, though under accepted rules its date is restricted to its use by Fabricius in 1775, has still a long precedence over nobilis (Schrank, iSoj). 2 For all three forms, as already stated, the correct -generic name is Potamobius. As in other inland counties, the remaining crustaceans are all of small size and of no commercial importance, though undoubtedly valuable as consumers of waste products and as a food supply to animals higher in general esteem. Conspicuous among the minor species is the 1 Huxley, The Crayfish, ed. 3 (1881) p. 297. Faxon, Proceedings of the American Academy of Am and Sciences (1884), xx. 153-6. 126