ers. The organs of mastication are not however the only organs that are worthy of being noticed. The beautiful and delicate structure of their feet and branchial appendages are worthy of all admiration. These latter organs are almost constantly in motion, and present a most interesting appearance when viewed under the microscope. The extraordinary method they have of reproducing their young, with the transformations which some of them undergo in an early stage, arc subjects which might occupy a considerable time in describing, and which cannot be attended to without exciting the greatest interest in the mind of the observer. A full and most interesting account of these curious matters may be seen in Jurine's work on the Monoculi found in the neighbourhood of Geneva, and in a series of papers published in the 'Magazine of Zoology and Botany' for 1836–7.
The least known of all the genera of this interesting order of creatures is the genus Cythere. It was established by Muller, in his 'Zoolog. Dan. Prod.' in 1776, and several species were afterwards described by him in his 'Entomostraca,' in 1781. The whole of these were marine, and no other author, after Muller, seems to have taken any notice of them, either with regard to their anatomical structure, or extending the number of species. Nestling in the quiet secluded little pools in the rocks on the sea-shore, amongst the little corallines and sea-weeds which make their abode there, they were very likely to escape the notice of most naturalists; and accordingly, as far as I know, no new recent marine species had been described since Muller's time, till I described several in the 'Transactions of the Berwickshire Naturalists' Club ' for 1835; and afterwards in the 'Magazine of Zoology and Botany,' in 1836: where also I have described all that is yet known of the anatomical structure of these curious little creatures.
In 1817, Say, in a paper published by him in the 6 Journal of the Academy of Natural Sciences of Philadelphia,' on the Crustacea of the United States, recorded a species as found in fresh water in Georgia and East Florida, and we believe that till now it was the only recent fresh-water species that had been discovered. In the 'Transactions of the Geological Society,' v. 136, Mr. Sowerby describes a fossil species from Hampstead; but there is so little difference in the shell between this genus and Cypris, that it appears to me very difficult to be able to refer a fossil species to the genus. In the autumn of 1841 I discovered the species here described, in a pond of fresh water near Hanwell, in Middlesex, being the second fresh-water species yet recorded.
