H. Rathke on the Metamorphosis of the Crustacea. 263 i s doubtful to what genus it should be referred ; most probably to my genus Thelmatophace. In Steudel's Nomenclat. Bot. there is a plant Lemna punctata, Meyer, of which I am ignorant. Lemna obcordata, P. Beauv. and Vahl. as well as Lemna dimidiata, Rafin., are erroneously enumerated amongst the Lemnacece, since an inspection of authentic specimens from the authors themselves proves them to be species of Riccia. XXX. — Contributions to the History of the Development of the Decapod Crustacea. By Heinrich Rathke*. One of the objects which I had proposed to myself for my tour through Scandinavia and Denmark, was an investigation of the Crustacea as regarded their development. Of Deca-pods which might serve as subjects for this investigation, se-veral, it is true, fell in my way ; fewer, however, by far than I had expected : these were Astacus marinus, Pagurus Bern-hardus, Galathcea rugosa, and a crab, which I consider to be Hyas Araneus. The details respecting these I design to make known, together with the results of the examination of various other animals, in a separate work ; as, however, some time may elapse before its publication, I will here communicate the most essential particulars of what I have learned respecting the development of the above-mentioned Crustacea, in order, as soon as possible, to record a testimony to the correctness of Thompson's discovery, that even the Decapods, after they have already quitted the egg, undergo a very considerable metamorphosis. 1. Astacus marinus. — Embryos just on the point of hatch-ing, possess already five pairs of feet, and these are similar in form to those of the full-grown specimens. But to the coxae of each is attached a part representing a narrow and long ap-pendage of the leg, proceeding down it on its outer side, little inferior to it in length, and composed of two larger members, of which the inferior one again consists of ten smaller articu-lations, and carries a number of long bristles. The same also is the case with the foot-jaws (Kieferfussen) of the second and third pair, of which, moreover, the hindermost is even at this period the largest of all, and it is evident from this that the above appendix represents the subsequent palpus flagelliformis. The four posterior foot-jaws and the ambulatory legs have also in general a resemblance to the legs of the Schizopoda, especially to those of Mysis. But this similarity afterwards * From Wiegmann's Archiv. (Part III. 1840.) — Translated and com-municated by Mr. W. Francis.