The occurrence of two kinds of males in the same species may perhaps not be a very rare phenomenon in animals in which the males differ widely from the females in structure. But only in those which can be procured in sufficient abundance, will it be possible to arrive at a conviction that we have not before us either two different species, or animals of different ages. From my own observation, although not very extensive, I can give a second example. It relates to a shore-hopper ( _Orchestia_ ). The animal (Fig. 7) lives in marshy places in the vicinity of the sea, under decaying leaves, in the loose earth which the Marsh Crabs ( _Gelasimus, Sesarma, Cyclograpsus,_ etc.) throw up around the entrance to their borrows, and even under dry cow-dung and horse-dung. If this species removes to a greater distance from the shore than the majority of its congeners (although some of them advance very far into the land and even upon mountains of a thousand feet in height, such as _O. tahitensis, telluris,_ and _sylvicola_ ), its male differs still more from all known species by the powerful chelæ of the second pair of feet. _Orchestia gryphus,_ from the sandy coast of Monchgut, alone presents a somewhat similar structure, but in a far less degree; elsewhere the form of the hand usual in the Amphipoda occurs. Now there is a considerable difference between the males of this species, especially in the structure of these chelæ—a different so great that we can scarcely find a parallel to it elsewhere between two species of the genus—and yet, as in _Tanais,_ we do not meet with a long series of structures running into one another, but only two forms united by no intermediate terms (Figs. 8 and 9). The males would be unhesitatingly regarded as belonging to two well-marked species if they did not live on the same spot, with undistinguishable females. That the two forms of the chelæ of the males occur in this species is so far worthy of notice, because the formation of the chelæ, which differs widely from the ordinary structure in the other species, indicates that it has quite recently undergone considerable changes, and therefore such a phenomenon was to be expected in it rather than in other species. Figs. 8 and 9. The two forms of the chelæ of the male of Orchestia Darwinii, magnified. Figs. 8 and 9. The two forms of the chelæ of the male of _Orchestia Darwinii,_ magnified. I cannot refrain from taking this opportunity of remarking that (so far as appears from Spence Bate’s catalogue), for two different kinds of males ( _Orchestia telluris_ and _sylvicola_ ) which live together in the forests of New Zealand, only one form of female is known, and hazarding the supposition that we have here a similar case. It does not seem to me to be probable that two nearly allied species of these social Amphipoda should occur mixed together under the same conditions of life.