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WISZNIEWSKI in the neighbourhood of Lakę Wi<>ry and the spe-cimens of G. substriatus Steph. used for comparison were colle-cted at the same time and in the same neighbourhood. This materiał was collected without selection and was all preserved in alcohol. When I received it I sorted the individuals into two groups on the single character of the colour of the mesosternum. In this freshly preserved materiał no difficulty arose; the diffe-rence between the black mesosternum of G. nałator and the red mesosternum of G. substriatus was quite obvious even when the latter species had the mesosternum infuscated. The whole of the specimens with the black or very dark brown mesosterna had the elytral striae almost uniformly deeply punctured and were definitely smali. The largest c’ and the largest 7 were both below the average size of the same sex of G. substriatus Steph. This I believe indicates that the species are truły dis-tinct. The fact that a specimen of G. substriatus was as smali as the smallest G. nałator is not surprising for smali individu-als of almost every species of Gyrinus, and indeed of every species of living creature, are not uncommon. The difficulty of making -accurate measurements is considerable. Measurements madę by different workers cannot be safely compared nor, indeed, can measurements madę by the same worker with different instruments. These measurements were madę conse-cutively over a period of less than three weeks, using an exactly similar techmąue. They are therefore directly comparable and there is no likelihood of experimental error affecting the averages frorn a comparative standpoint.

In separating this species from G. substriatus the black mesosternum is the most valuable character. It is not however absolutely diagnostic for there is a variety of G. substriatus (var. fowleri O.-C.) which has the mesosternum very deeply infuscated or black. The uniform puncturation of G. nałator and the smaller size are characters of value. G. natator is also slightly morę elongated on the average than G. substriatus. The difference in length and width of the median lobe of the aede-agus is not very easily seen, especially in dried specimens. In G. substriatus the parameres are usually red or yellow with a patch of infuscation apically, while in G. natator the parameres are uniformly reddish brown. When a series of half a do-