The fresh-water Mussels, or the family of the Unionidæ, have received a considerable share of attention from zoologists. The facility with which the different species may be obtained, and the highly interesting embryonic forms of these molluscs, whether during the time they remain within the folds of the external branchiæ, or after they have been excluded, render them favorite subjects for investigation. The whole question of the development of the fry is one of considerable interest; but it appears that all investigations that have hitherto been published on this point relate to their intra-branchial existence, while we are left entirely, I believe, without information on the condition and development of the young Anodontas during the period that elapses between their exclusion and their assumption of those characters that belong to the mature animal. The young Anodonta, at the time it is ready for exclusion, bears a form very different from that which it is destined ultimately to assume. This, no doubt, holds good in the case of the other British members of the group; it is certainly true of Unio pictorum, which, however, appears to be later than the Anodonta in parting with the fry. So different, indeed, is the form of the young animal during its intra-branchial state, that M. Rathke,* in 1797, actually regarded it, not as the young mollusc, but as a veritable parasite, to which he gave the appropriate name of Glochidium (γλ ω χí ς), in allusion to the two curious serrated points or hooks which it possesses. Rathke’s opinion was endorsed by M. Jacobson, in a paper published in the Danish ‘Transactions’ in 1828. † But the real nature of the bivalves found in the branchiæ of Anadonta and Unio was fully proved by C. G. Carus in 1830.‡ In his valuable memoir on this subject, which may be regarded as the fountain-head of all that is really known concerning the anatomy of the Glochidium-embryos, the true relations of these creatures to their parent are clearly made out. It also contains observations upon many points of considerable general interest with respect to the structure and early stages of development of the molluscan ovum; and it is in this paper that Carus for the first time clearly describes the curious phenomenon of the rotation of the embryo within the egg-membrane, although it had doubtless been observed in the ovum of some species of Unionidæ by Leeuwenhoek, and was also noticed by M. Bauer; who absurdly attributes the movement to the incessant attacks of an invisible worm, which he imagined was devouring the vitellus.