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CRUSTACEANS Though Surrey has been for an indefinite period within easy reach of competent naturalists, it has not tempted any of them to make a systematic investigation of its carcinological fauna. The present chapter derives benefit from the researches of Dr. Baird and a few others, rather because they happened to carry them on in the neighbourhood of London than because they had any special intention of exploring this county. Nevertheless the area in question undoubtedly contains species of no little interest in several divisions of the crustacean class. It will also appear from some discoveries that many more may reasonably be expected to follow. In certain parts of the world the higher groups of the Malacostraca are well represented both on land and in fresh water. But this is not the case in England. Of the Brachyura or * short-tails ' we have no terrestrial or fluviatile species, no land crabs or river crabs. Here also the fresh water yields but a scanty supply of Macrura or ' long-tails.' Of these Surrey, like many other inland counties, can only claim with confidence a single species, the common river crayfish, Potamobius pallipes (Lereboullet). 1 In reference to this species two questions are not uncom- monly mooted, one concerning the difference between a crayfish and a crawfish, the other concerning the difference between these and a lobster. All three were in uncritical ages included in a common genus Astacus, whereas now they are all three generically distinct. Crayfish and craw- fish however are essentially the same word, and like other vernacular names have the misfortune to be sometimes used interchangeably. In drawing any distinction therefore it is important first of all to nail the name crayfish hard and fast on to the fluviatile species (Potamobius^ the river-liver), reserving the other name for the marine animal sometimes called a rock-lobster. This, the crawfish, will be found to stand out boldly and clearly distinct by its far greater size, by its roughened carapace, by its exceedingly long and stiff second antennas, and by its want of well fashioned claws or nippers. The crayfish, on the other hand, is very like a little lobster, but it will be found to differ from the true lobster (Astacus gammarus) by having a larger ' scale ' to its second antennae, and by having the segment which carries the last pair of trunk- legs movable upon the preceding segment instead of being firmly fixed to it. There are also small distinctions easy to observe in the frontal process known as the rostrum and in the caudal extremity known as the 1 For the information that this is found ' in the Wey about 100 yards above Harris's boathouse at Weybridge' I am indebted to Henry F. Field, Esq., writing from 17 Argyll Road, Baling, March 14, 1901. I8 7