In September 1958, Prof. J. E. Harris, F.R.S., treated freshly collected Eddystone shell gravel with a magnesium sulphate solution and so produced a rich sample of the microfauna. This has never been adequately investigated, though the macrofauna is well known. The animals collected were mostly small Malacostraca, in which Amphipoda predominated, and Prof. Harris kindly passed on to me the whole sample to examine. I have found in it thirty-four species of amphipods, isopods, tanaids and cumaceans of which no fewer than fifteen are additions to the Plymouth fauna list (see Marine Biological Association, 1957). Some indeed are new to British waters, three or four being evidently undescribed species. One of the most striking of these is described here. The collection as a whole will be reported on later.