THE GLOBE PARTITIONED BY CRAYFISHES

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liarity that has long been known in the genus Cambarus. It was at one time supposed that one of the forms, not much differentiated from the female, might be sterile, and that the more highly developed and specialised form was the fertile male. But Mr. Faxon, having kept some specimens of the latter form under observation, found that after pairing at the next exuviation they assumed the less differentiated form, and his inference has been generally accepted that the two forms alternate in the same individual during a certain part of its life. As it is not probable that the Potamobiidae have a monopoly of this curious changefulness, the chance of its occurrence is one more pitfall to be guarded against in the institution of new species.

Family 4.-Parastacidae.

These agree very closely with the preceding family except in regard to the branchiæ, appendages of the pleon, and the telson. Here the first maxillipeds have the epipod almost always provided with a certain number of well-developed branchial filaments; the podobranchiæ of the following appendages are devoid of more than a rudiment of a lamina, while some of their filaments and attendant setæ terminate in hooks. The first segment of the pleon has no appendages in either sex, and the appendages of the four following segments are large. The telson is never divided by a transverse hinge.

To this family there are allotted six genera, all belonging to the Southern hemisphere, and living, like those of the preceding family, only in fresh or brackish waters. The facts of distribution in regard to the two families are remarkable. Several species of Potamobia are found in rivers of Europe and Asia, and five species of that genus exist in rivers of North America, west of the Rocky Mountains, whereas fifty-two species of Cambarus inhabit the rivers and lakes of North America east of that range. Of the Parastacidæ, Astacoides, Guérin, 1839, with its solitary species madagascariensis, is found only in Madagascar; Parastacus, Huxley, 1878, was established for

species that belong to Southern Brazil; Paranephrops, White, 1831, is found only in New Zealand and (possibly) Fiji, while even within the limits of New Zealand its two species, planifrons and zealandicus, are found by Mr. Chilton to have distinct and separate ranges; Astacopsis, Huxley, 1878, and Engaus, Erichson, 1846, belong to Australia and Tasmania, while Charaps, Erichson, 1846, belongs to Australia alone. Charaps was instituted only as a subgenus by Erichson, and by Mr. Haswell it is united with Astacopsis. Spence Bate, in remarking upon the peculiarities of distribution here set forth, speaks of the several genera being adapted each to its own locality, no two genera being known to exist in one habitat,' but to support this statement he assigns Astacopsis to Australia, Engaus to Tasmania, and Charaps to Van Diemen's Land, intending perhaps a just reproach to those who altered the name of Van Diemen's Land into Tasmania. In fact the small burrowing Engaus may be peculiar to that island, and, if not, it is separated by rather subtle distinctions from Astacopsis, so that the three genera in question form a very united group, and it is singular that, while they agree together in their branchial arrangement, they differ in that respect from all the other genera in the two families under discussion. The unnamed Australian Crayfish' of Huxley, which sometimes reaches a length of twenty inches, is pronounced by Mr. Haswell to be Astacopsis serratus (Shaw). In Paranephrops zealandicus, Mr. Wood-Mason has observed that the young are specially fitted for attachment under the pleon of the mother. The specimens examined were under a third of an inch in length. The two hindmost pairs of legs have the sixth joint provided at its extremity with a strongly hooked, exceedingly acute, movable claw, and on the lower edge at the end with six or seven sharp spines, against which the claw folds, and thus forms a very efficient prehensile arrangement.'

A SMALL TRIBE

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CHAPTER XV

TRIBE V.-STENOPIDEA

THE carapace is produced to a laterally compressed rostrum. The first antennæ have two flagella, the second have a scale. The mandibles have a three-jointed palp. The exopod of the third maxillipeds is small, slender, and almost rudimentary. The first three pairs of trunk-legs are chelate, the third pair being the longest and largest. The branchiæ are filamentous; only the second maxillipeds have a podobranchial plume; the hindmost pleurobranchial plume is the largest. The first pair of pleopods is onebranched and foliaceous; the uropods and telson have no

transverse suture.

Family Stenopida.

This being the only family has the characters of the tribe. It contains two genera long included among the Penæida, with which they agree in having the third pair of trunk-legs larger than the two preceding pairs, but separated from that group by the structure of the branchiæ. Of the third genus now transferred to this family, the branchiæ have not been described.

Stenopus, Latreille (in Desmarest), 1825, has a long, flat, obtusely pointed scale on the second antennæ, the third trunk-legs long and slender, the fourth and fifth pairs with the antepenultimate joint subdivided, the telson tapering. The genus ranges from the eastern to the western hemisphere and from the Arctic regions to the tropics. Stenopus hispidus (Olivier) is recorded from the Pacific, from Bermudas, and perhaps from Greenland. Spence Bate's figures of this species are reproduced on a re