ambo 2-articulati. Caput breve, infra elongato-productum, antennas anticas obsolescentes versus apicem gerens.
CERIODAPHNIA.
Pedes foliacei numero decem. Corpus fere globosum, capite brevi. Antennæ anticæ minutæ (raro elongatæ?). Testa cellulis hexagonis et pentagonis subtilissimè areolata.
PLEOPIS.
Caput grande, oculis repletum. Pedes numero octo. Corpus postice, non deflexum, fere rectum; abdomen crassum extremitate furcatum, setis nullis. Testa postice rotundata. Rami antennarum posticarum 3-articulati.
The whole family I should propose to divide into three sub-families, according to the number of the legs, almost in the manner suggested by Prof. Dana. Firstly, Sidinæ, with six pair of foliaceous legs, and containing the genera Sida, Holopedium, Latona, and Penilia. Secondly, Daphninæ, with five pairs of legs, and divided into the three genera, Daphnia, Bosmina, and Lynceus. And, thirdly, Polypheminæ, characrized by four pairs of legs, and an immense eye. The latter sub-family would comprise Polyphemus, Bythotrephes, Podon, Evadne, and Pleopis.
Prof. Dana is, however, wrong in describing the head of Polypheminæ as "oculis repletum," the part thus occupied being, as Prof. Leydig correctly points out, only the anterior part of the head.
On the whole, Prof. Leydig's monograph is worthy of the reputation of its author. The specific descriptions are drawn up with care, and in much detail; the drawings are clear and good: and though I have not hesitated freely and frankly to indicate the points on which, in my opinion, his views are incorrect, or his arguments inconclusive, I am glad to take this opportunity of thanking him for what is undoubtedly a valuable work on a very interesting subject. There still remain, how- ever, unsolved, many most important questions with reference to the anatomy and development of Daphnia; and especially an examination into the embryology of the ephippial, as compared with that of the ordinary eggs, would, doubtless, well reward any one who would undertake it. In a natural condition, indeed, the ephippial egg is so opaque, that its internal condition cannot satisfactorily be ascertained; but it is probable that this difficulty might be overcome by the use of glycerine, or some other chemical re-agents. The light which is in this manner thrown upon tissues is most surprising; and it is probable that the future progress of histology will depend, at least, as much on the judicious use of chemical agents, as upon that of the microscope itself.
