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 CRUSTACEANS very large number of species and great swarms of individuals in fresh water, of which the smallest tracts are sometimes the most productive. Of the Malacostraca our inland representative highest in rank and most important by far in point of size is the river crayfish Potamobius pallipes (Lereboullet). At Matlock a fishmonger and others of whom inquiries were made replied that they had never heard of this species as living in Derbyshire. But on applying to Dr. H. C. Sorby, F.R.S., I learned from him that a relative of his had formerly spoken to him of its being found in the river Wye. Dr. Sorby also took the trouble to write on the subject to Mr. John Hall of Norbury, Sheffield. Mr. Hall in a letter dated October 21, 1901, says : ' All I can tell you about cray- fish is that some dozen years ago I caught one at Grindleford Bridge while fishing for trout in a flood with wasp grub. The keeper told me they were often caught there in a flood, but by no means common.' With reference to this letter Dr. Sorby subsequently wrote to me saying, ' Grindleford Bridge is on the Derwent, a few miles below Hathersage and a good many above Chatsworth. It is a station on the Midland line.' In an earlier note he says : ' I send you Mr. John Hall's letter, which gives unimpeachable evidence of the crayfish in the Derwent. He is a gentleman of eighty or so, and has been a fisherman in Derby- shire most of his life. He has taken much interest in natural history and was a friend of Frank Buckland. Being thus found in the Derwent we may conclude that my aunt was correct in saying that it [the cray- fish] occurred in the Wye. These are the two chief rivers of the county.' The Rev. H. S. Gorham, F.Z.S., has also assured me by word of mouth that when he lived in the neighbourhood of Ashbourn crayfish were plentiful in the banks of the river Dove. As the result then of these inquiries it may be fairly inferred that the species certainly belongs to the county fauna, but that it is not everywhere especially common. Its position in the crustacean class may be briefly explained. The highest Malacostraca are divided into Brachyura and Macrura, short-tails and long-tails. Potamobius belongs like the lobster and the shrimp to the latter section. Like the lobster and the shrimp it has a pair of movable eyes, two pairs of antennae, six pairs of jaws, five pairs of trunk-legs, and an abdomen or tail of seven segments, of which each except the last carries a pair of appendages. Thus includ- ing the eyes there are in all twenty pairs of appendages, and assuming that each of these implies an originally distinct segment supporting it, we shall have the malacostracan body composed of twenty-and-one seg- ments. Since there are some orders in which the eyes are not movable and have no proved claim to a segment of their own, the first or ocular segment is, like the last which is devoid of appendages, in a somewhat exceptional position. Between these two extremities however there are nineteen segments of the body which maintain themselves with a re- markable though not absolutely unbroken uniformity throughout the highly diversified and very extensive division of the Malacostraca. It will be noticed in the crayfish that the eyes and the following 103