§ I. Introduction. In the ‘Geological Magazine,’ dec. 2, vol. v. (March and June 1878), pp. 103 & 277, I offered some observations on the Ostraeoda of the Purbeck and Wealden Formations, and endeavoured to determine the species already described and figured by Sowerby, Fitton, F. A. Römer, and W. Dunker, and by E. Forbes, Lyell, and P. de Loriol. One of these species was further noticed (as Cythere ? purbeckensis ) in the ‘Proceedings of the Geologists' Association,’ vol. viii. 1883, p. 58, together with an additional species which I had defined as Cythere boloniensis [ bononiensis ] in the Bull. Soc. Géol. France, ser. 3, vol. viii. 1882, p. 616. Having been enabled, by the help of the Rev. O. Fisher, Mr. W. Cunnington, Mr. Horace B. Woodward, Mr. J. C. Mansel-Pleydell, Prof. J. F. Blake, and other friends, to add considerably to my own collection of Purbeck Entomostraca, among which are many specimens collected by Fitton and Brodie, and having been kindly aided in my practical work by Mr. E. T. Newton, F.G.S., and Mr. C. D. Sherborn, I have begun to examine and determine the whole series of Ostracodous species characteristic of the Purbeck-Wealden beds. As far as opportunities have offered, I have also studied the specimens preserved in other collections and museums. § II. T he O stracoda of the P urbeck B eds . Sources of Information. Professor Edward Forbes commenced this work of determining the Purbeck Ostracoda long ago; but his results were not fully recorded, nor, indeed, is it now