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Four pairs of legs, projecting beyond the carapace, which serves only as a receptacle for the eggs. Head separated from the thorax. The bristles on the large antennæ plumose. Post-abdomen produced behind as a long, cylindrical process, with two terminal setæ.

General characters as in Polyphemus, but the anterior legs much longer than the others. The post-abdomen terminating in a single, very long spine.

General characters as in Polyphemus, but the post-abdomen ending in two long spines.

General characters as in Polyphemus, but with the head and thorax united. Post-abdomen very short; the tail bristles quite small. The receptacle of the eggs very large, and produced into a point.

Although this classification suppresses several of the unnecessary genera which had been proposed by previous writers, it may be doubted whether the list ought not to undergo a still farther diminution. The number, arrangement, and size of the hairs on the antennæ afford, in Entomostraca, excellent specific differences; but they are surely not of sufficient importance to be used as generic characters. Although I think few naturalists now regard genera as being more than a convenient memoria technica, it is evidently desirable that the characters used to separate genera should, throughout the animal kingdom, be as nearly as possible of equal importance, and, to borrow a mathematical expression, of a higher "order" than those by which species are distinguished. Of course this rule can only be applied in a very rough manner, since it is almost impossible to estimate the relative value of different characters; but it must, I think, be admitted that, if applied, for instance, to the genus Acanthocercus, it would not justify the generic separation of A. rigidus from the species of the preceding genus.

In the same manner, the groups Macrothrix and Pasithea, so far as the characters given are concerned, are scarcely entitled to rank as separate genera.

On the other hand, we must add to the list two genera proposed by Prof. Dana, in his great work on Crustacea. A third, Ceriodaphnia, is scarcely distinct enough from Daphnia. I subjoin the characters given for all three, as many other naturalists may, like Prof. Leydig, have been unable to obtain Prof. Dana's book.

Pedes foliacei numero duodecim. Antennarum posticarum rami