Amongst a great number of crabs collected in Plymouth Sound for my studies of the central nervous system, one specimen was found with a very interesting abnormality. The thorax of the crab—a female—is normal. The length of the carapace is 47 mm., the breadth 64 mm. The claws and legs are in the right positions, and of normal proportions, with the exception of the fourth leg on the left side, which is smaller than usual. In my opinion, this leg was broken some time before the crab came in, and was not yet quite regenerated. The first four segments of the abdomen are also normal. Each has a couple of pedes spurii. To the fifth segment, which in the crab is always legless, a large leg is fastened on the left side. (Fig. 1.)