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Show 1 9 0 4 .] ASELLOTA-GROUP OF CRUSTACEANS. 3 0 5 pair of pleopoda. Second pair is wanting, as in all Asellota. In the male the first pair (PI. XX. fig. 2 g; PI. XXI. fig. 2 / ) is slightly longer and at the base narrower than the female operculum ; the sympods of the two appendages are completely fused with each other, forming a short, transverse plate; each pleopod has one ramus, which is free, oblong, and between two and three times longer than the sympod ; each ramus can be moved a little by a tiny muscle (m.) in the sympod. The second pair is a good deal smaller than the first; each appendage (PI. XX. fig. 2 h ; PI. XXI. fig. 2 g) consists of an oblong subtriangular plate, the sympod, the inner margin of which is sinuate, and from the distal end of this margin arise the two rami. The exopod is very small, slender, a little curved, scarcely hook-shaped, one-jointed, but in S. siamense (PI. XXI. fig. 2 h, ex.) a vestige of a division into two joints is observed. The endopod is rather long, very slender, two-jointed, and strongly geniculate in the articulation (PL XXI. fig. 2 h, en.) ; the proximal joint contains a muscle for the movement of the second, which has no internal cavity, while its end is obtuse and often furnished with a brush of exceedingly short bristles; especially in S. antillense, the terminal portion bearing this brush is distinctly marked oft' from the joint. The plate-shaped sympod contains muscles to the rami, two to the endopod, and at least one to the exopod. The two pleopoda of this pair touch each other at their base ; they are covered by the first pair. In both sexes the pleopoda of the fourth pair (Pl. XX. fig. 2 k) are similar as to size and structure ; each has a short, broad sympod and a two-jointed exopod, which is slightly longer and somewhat broader than the unjointed endopod, and adorned with plumose setae along the distal part of the outer margin; both rami are lamellar and both seem to be respiratory. The fifth pair (PI. XX. fig. 2 I) has no discernible sympod and only one ramus, which is large, unjointed, but otherwise shaped and adorned with setfe like the exopod of the preceding pair, and accordingly it is in all probability the exopod itself. Uropoda consist of an unjointed sympod and two unjointed nearly styliform rami; the exopod is as long as or longer than the sympod, nearly as long as or somewhat shorter than the endopod. III. Comparison between the Genera Stenetrium and Asellus. PI. 39 in Sars's work is filled with figures of Asellus aquaticus L. I can therefore refer to his good drawings, and give only a new figure of the second male pleopod. The essential differences between Asellus Geoff, and Stenetrium Hasw. are found in the antennse, the maxillipeds of the adult female, and some of the pleopoda. The peduncle of the antennae .shows the same number of joints in both genera, but in Asellus the exopod from the third joint is wanting. In Asellus the basal joint of each maxilliped possesses in the ovigerous female a rather large plate, bearing a number of bristles at the end and directed P r o c . Z o o l . S o c .-1904, V o l . II. No. XX. 20 |