common shrimp from Plymouth to be only 011. Perhaps this difference may in part be accounted for by the fact that the prawn is essentially a swimmer, while the shrimp, in confinement at any rate, spends most of its time standing or crawling on the surface of the sea-bottom. The prawn is certainly a more powerful and rapid swimmer. It swims backwards, propelling itself by means of the hinder part of the abdomen, the terminal portion of which is fanshaped, the fan consisting of the telson and the exopodites of the last pair of appendages. This hinder half of the abdomen starts from the extended position, and is brought, with a rapid stroke, up under the fore part of the abdomen and thorax, pushing the water before it, and so propelling the animal backwards. The 3rd seg ment acts as the hinge of the propeller, and it may be assumed that the exoskeleton is here exposed to the greatest strain, for here we find the highest degree of correlation, as is shown by the following figures: Index of correlation between terga of abdominal segments 1 and 2 4 = 0.71. 5 = 0.62. 6 and telson = 0.51. Professor Weldon determined three other values in the shrimp, viz., the relations between carapace length and post-spinous portion of carapace, between carapace length and tergum of abdomen vi., and between carapace length and telson. Their values were (in Plymouth specimens) 0.81, 0-09, and 0-18 respectively. It is not possible to institute a comparison with exactly the same organs in the prawn, as, owing to the great variation in the length of the rostrum, no appreciable degree of correlation exists between the whole carapace including rostrum (measurement No. III) and the other organs. But if we take measurement No. II (from orbit to, hinder edge of carapace) as an approximate equivalent to the carapace measurements of the shrimp, we find in the prawn the corresponding values are 0·59, 0·59, and 0.40. The difference in the first measurement, viz., shrimp, 0·81; prawn, 0·59, may be accounted for by the fact that the dorsal spines probably do not correspond in nature or function in the two animals. The shrimp has but one median dorsal spine, situated far forward; the prawn has a row of them. But why the prawn should exhibit a so much higher degree of correlation between the carapace and the two terminal segments of its body than the shrimp is not, I think, readily to be ex plained at first sight, unless it be due to difference in habits of locomotion above referred to. The subjoined table furnishes the necessary data for constructing approximately the curve of each of the separate organs. The Roman numerals in the first column refer to the organ described above under the corresponding numbers on p. 235; the second column contains the number of animals out of the whole sample of 1000 in which a measurement of the particular organ was made; the third column contains the median value; and the fourth the "probable error" or quartile deviation from the median of each organ expressed in thousandths of the body length; the fifth and sixth columns refer to the same animals after subtracting from their number thirteen individuals in which the rostrum or telson was deformed, and for these the arithmetical mean and probable error (obtained by finding the mean error and multiplying it by 0.845) are given. All the figures in columns 3 to 6, inclusive, represent thousandths of the body length, except those standing against organ No. I (the body length itself), which represent millimetres. 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