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Show MR. F. 11. A. MARSHALL ON Text-fig. 1. [Jan. 14, Genital apertures of Nephrops norvegicus. Male of Nephrons norvegicus having abnormal genital apertures on each of the third and fourth walking-legs. Genital apertures of Nephrops norvegicus. Male of Nephrops norvegicus having abnormal genital apertures on the right third and right fourth walking-legs. In no specimen was either of the normal apertures wanting. The animals were examined in batches-the first batch, which was much the largest, consisting of the stock of Norway lobsters in the zoological laboratory of the University of Edinburgh, obtained for the use of students during the summer session. The rest were procured at various intervals of time during the summer and autumn. The numerical details of these batches are now given :- (1) Females 56 Normal males 485 Abnormal males with arrangement B... 231 Ditto with arrangement C 21 D 17 >63 » » E 1 | H lj 1 >548 Total ... 604 |