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 CRUSTACEANS or observes with interest and pleasure. Some even in the scientific world think that such names are of no importance. Some discuss them with an almost passionate eagerness. In truth one name may do as well as another so long as the meaning is really known. But there a diffi- culty comes in. Just as the Syrian captain thought that the rivers of his own Damascus must be of more virtue than all the waters of Palestine, we are apt to think that expressions like ' river crayfish,' drawn from the ' well of English undefiled,' must be superior to any terminology of uncouth and sometimes barbarous Latin. Unfortunately the term 'river crayfish ' is indifferently applicable to any one of several scores of species distributed over Europe, Asia, America and Australia. Out of the whole number only one species of one genus is known to occur in England, and that one is not Astacus fluviatilis. The generic name Astacus, formerly applied with great vagueness to many very distinct creatures, is now properly confined to lobsters which live in the sea. In Potamobius, a word meaning ' life in a river,' our freshwater crayfishes enjoy an ap- propriate designation. It is the only genus of them found in Europe, and its three or four European species are all closely connected. Between two of these Huxley institutes a very interesting comparison, to illustrate the difficulty of deciding whether differing forms should in certain cases be regarded as distinct species or as mere varieties. He says : If large series of specimens of both stone crayfishes and noble crayfishes from different localities are carefully examined, they will be found to present great variations in size and colour, in the tuberculation of the carapace and limbs, and in the absolute and relative sizes of the forceps. The most constant characters of the stone crayfish are : 1. The tapering form of the rostrum and the approximation of the lateral spines to its point ; the distance between these spines being about equal to their distance from the apex of the rostrum (fig. 61, A). 2. The development of one or two spines from the ventral margin of the rostrum. 3. The gradual subsidence of the posterior part of the post-orbital ridge, and the absence of spines on its surface. 4. The large relative size of the posterior division of the telson (G). On the contrary, in the noble crayfish : 1. The sides of the posterior two-thirds of the rostrum are nearly parallel, and the lateral spines are fully a third of the length of the rostrum from its point ; the distance between them being much less than their distance from the apex of the rostrum (B). 2. No spine is developed from the ventral margin of the rostrum. 3. The posterior part of the post-orbital ridge is a more or less distinct, some- times spinous elevation. 4. The posterior division of the telson is smaller relatively to the anterior division (A). I may add that I have found three rudimentary pleurobranchiae in the noble cray- fish, and never more than two in the stone crayfish. 1 With these contrasted details a student would find it a useful exer- cise to determine for himself whether our English species is the stone crayfish or the noble crayfish. Most of the technical terms are not difficult to understand. The rostrum is the median projection from the 1 Huxley, The Crayfish, ed. 3 (1881), pp. 294, 295, fig. 61, A, G, B, H, on p. 233. 125