Percnon pilimanus (A. Milne-Edwards).

Acanthopus pilimanus A. M.-Edwards, Nouv. Archiv. Mus., 9, p. 300, pl. 14, fig. 5, 1873. Leiolophus pilimanus Miers, Ann. Mag. Nat. Hist., ser. 5, 1, p. 154, 1878. Percnon pilimanus Rathbun, Bull. U.S. Fish. Comm. for 1903, p. 842, 1906.

Two males and several females from Sunday Island.

These specimens agree very closely indeed with Milne-Edwards's description, except that there is no large tuft of fine hairs on the propod of the chelipeds; a well-marked tuft is, however, present on the merus of the larger male. The width of the abdomen of the larger male at the base is just equal to that of its length to the base of the last segment; in the smaller male the width is rather greater than this.

It is possible that these specimens should be referred to P. planissimus, but I assign them to P. pilimanus owing to the slightly narrower abdomen, the spines on the inner margins of the antennulary cavities, and to the fact that they agree minutely with Milne-Edwards's description and figure except as regards the hairs on the propod of the chelipeds. The smaller specimens agree well with the description of P. planissimus given by Alcock (Journ. Asiatic Soc. Bengal, 69, p. 439), except that the second row of spinules on the merus of the legs is well marked on the third legs as well as on the first and second; in the larger specimens the row is also indistinctly marked on the fourth. Miss Rathbun records both species from the Hawaiian Islands without comment.

Milne-Edwards states that the hairs on the chelipeds act as a sponge to maintain the humidity at the orifice of the branchial chamber. He had, however, seen only one male specimen, and, as the tuft on the merus is very small or quite absent in my female specimens, it seems more likely that the hairs may be a sexual character, developed in the adult male only, and in that case may not yet be fully developed in the two males in my possession. The females resemble the male except in the much smaller size of the chelipeds, which are much shorter and have the propod only slightly widened. The merus bears only a very small tuft of hairs in the larger female specimens, and none in the smaller specimens. It seems likely, therefore, that the tufts of fine hairs on the merus and propod are a secondary sexual character, developed only in large males, or perhaps only during the breeding season; they were evidently not present in the adult males of P. planissimus examined by Alcock, for he makes no mention of them. If, as seems likely, the other characters—i.e., the narrower abdomen and the spines on the inner margin of the antennulary cavities—. do not prove to be constant, one would be tempted to suggest that P. planissimus and P. pilimanus form one species, in which the males may develop the tufts of fine hairs on the chelipeds at certain seasons only.

Male: Width of carapace, 29 mm.; length, 32 mm.: total length of propod of cheliped, 15 mm.; width, 11 mm. Largest female: Width of carapace, 28 mm.; length, 31 mm.: total length of propod of cheliped, 8 mm.; width, 6 mm.

Mr. Oliver makes the following observations on the habits of this species: “Fairly common among rocks near low-tide mark. Very quick in its habits. Its colour somewhat resembles the rock, on which it stays perfectly still, but when any one approaches too near it darts into the water. When, after continued westerly winds, sand was driven ashore so as to bury the boulders on the north coast of Sunday Island to about half-tide