Art. XX.–Description of a New Species of Isopodous Crustacean (Idotea).

By Gerorge M. Thomson.

(Read before the Otago Institute, 26th November, 1878.)

The animals forming the genus Idotea are readily distinguished from other Isopods by having the segments of the abdomen more or less coalescent, and fewer than the normal number, seven, and particularly in having the “terminal segment very large, its appendages greatly developed, covering the whole inferior surface of the abdomen, and closing like doors over the branchial appendages.”

Of the three species described in Miers' Catalogue of N. Z. Crust., pp. 92, 93, the first, I. argentea, Dana, is a doubtful New Zealand species, having been obtained by Dana near New Zealand. It is found on the Australian Coasts, and also at Borneo, and is probably common in the West Pacific. The second species I. affinis, M. Edw., is common on our coasts, and seems to be the same as a species found on St. Paul's Island, and at the Cape. The third, I. elongata, Miers, has only been found hitherto at the Auckland Islands. The species which I now propose to add, is chiefly remarkable for its habitat, being the only species, as far as I can find out, occurring in fresh water. Numerous specimens were found by Prof. Hutton in the Tomahawk lagoon, near Dunedin. They were creeping about under stones, and appeared to be feeding on the ova of a fish, probably Galaxias, sp., which was found abundantly in the same locality. Whether they occur permanently in the fresh water, or only come up when a very high tide renders communication with the lagoon possible, I cannot say. The species is a distinct one, and has not hitherto been found on our coasts.