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Rh ler, and Acroperus harpæ, Baird, both procured in Stoke Park, and belonging to the genera with the head keeled on the top, are distinguished one from the other by the tail of the female. This in the former genus is long and slender and armed with spines on the hinder or upper margin, but in the latter is ' of moderate length and breadth, without spines on the aforesaid margin, and only furnished on the sides with uniform fascicles of very minute spines.' No very large bowl is needed for constructing a miniature pond in which to observe the manners and customs of these creatures. Their modes of motion are very various, and familiarity with these may be utilized for determining the kinds present in an aquarium. Mr. Scourfield says: ' Of all the peculiar modes of existence, that ef deliberately making use of the ceiling of a pond, i.e. the surface-film of water, for support, is probably the most remarkable. So far as is known, only a very few species have acquired the power in a fully developed fashion, and these are all included in two genera, namely Scapholeberis (Cladocera) and Notodromas (Ostracoda).' In regard to S. mucronata he explains that on the perfectly straight and flattened ventral margin of each valve there exists a series of very peculiarly modified setae, the anterior and posterior members of which are larger and project somewhat more than the rest; when the animal, which habitually swims in a reversed position, brings its ventral margin into contact with the surface of the water, the setas which project farthest from the shell pierce the surface-film and produce minute capillary depressions. These depressions, it appears, are large enough to support the difference in weight between the animal's body and the water which it displaces. By careful observation and experiment Mr. Scourfield has also determined that in addition to the morphological distinctions there is also a fundamental difference in the swimming habits of Daphnia and Ceriodaphnia on the one hand, and Simosa and Scapholeberis on the other, for whereas the two former always swim either vertically, or obliquely back uppermost, the two latter always swim more or less obliquely back downwards. Of Chydorus sphæricus Baird observes that its motion through the water is more like rolling, as Jurine describes it, than swimming.

Of the Ostracoda, so abundant everywhere both in species and individuals, there is for the moment only a single Buckinghamshire record, the common Cypris fuscata, Jurine, having been taken in a pond on a common near Burnham Beeches.

Of the Copepoda there are five species included in three genera, a small collection but covering rather a wide space in classification. Diaptomus castor (Jurine) occurred together with the Ostracode just mentioned. It belongs to the extensive family of the Diaptomidas, which is comprised in the division of the Copepoda called by Giesbrecht Gymnoplea. To explain this expression, it must be pointed out that in this order of animals the body is theoretically divisible into eleven segments,