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Show 1900.] H0M030SIS IN THE GENUS ASELLUS. 269 a number of Aselli taken from a large aquarium in the Cambridge Zoological Laboratory for examination by the students. As the figure shows (fig. 1), the abnormal structure stands exactly as the left antennule should. Both antennae and the right antennule are normal, as also are the mandibles, the other mouth-parts, and the rest of the body and its appendages, so far as they could be observed. The animal was of good size. Fig. 1. Head of Asellus aquaticus, seen from dorsal side.- rt. ant.1, right antennule; rt. ant.3, I. ant.2, right and left antennae ; rp, palp of normal right mandible projecting from below; p', palp of the abnormal mandible; trit, triturating process. The abnormal appendage (fig. 2, p. 270) is a fairly well-formed mandible. The blade is complete, having two toothed processes and eleven plumose setae. The latter are not quite regularly placed. The presence of two toothed processes proves that the appendage is a true left appendage, for the right mandible in Asellus aquaticus has only one toothed process, which bites between the two toothed processes of the left mandible l. The teeth on these processes are more numerous and less regular than in a normal mandible. 1 Sara, G. O., Crustacea d'Eau douce de Noryege, p. 97, pi. viii. fig. 26. |