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but it happens that some species which are common enough in the British seas, though unknown to Sars when his memoir was written, are exactly intermediate in character, presenting different combinations of those peculiarities which were relied on to separate Cythere from Cythereis. It is therefore necessary either to constitute two or three new genera for the reception of these aberrant forms, or to give a more extended signification to the original genus, so as to include all under the one term Cythere. The latter is the course which I have adopted. The intermediate species here referred to are C. albomaculata, convexa, and rubida. C. convexa exhibits a remarkable approach to the genus Bairdia in general outline, the two valves being very unequal and decidedly beaked behind. In all essential points, however, it is a true Cythere. This genus includes a very large proportion of the fossil species; its preponderance appears, indeed, to have been greater during the earlier periods of the earth's history than now, though possibly this may partly arise from the great thickness and durability of the shells of many species, and especially of many of the fossil forms which have thus been preserved, while other more fragile species may have been destroyed.

The Cytheres have no power of swimming, and are met with abundantly both amongst the fuci of the littoral zone and amongst the mud and sand of the deep-sea bed. A muslin or crinoline net used amongst the rock-pools of any part of our coast cannot fail in the summer months to capture numbers of them. In these situations C. albomaculata, lutea, viridis, and villosa are perhaps the commonest; while beyond the littoral zone we most frequently meet with pellucida, tuberculata, lutea, etc. The forms here named acerosa and semipunctata seem to be very rare. Their anatomy is not at all known, but their external peculiarities lead to the belief that they may constitute the types of new genera.

LIMNOCYTHERE, nov. gen.-Animal like Cythere, except that the upper antennæ (Fig. 7) are armed with short setæ instead of spines. They are five-jointed, slender, the antepenultimate joint excessively short, terminal joint much elongated. Shell rather thin, irregularly tuberculate or spinous. Inhabits fresh water.

L. inopinata (Baird); monstrifica (Norman).-I have not yet been able completely to examine the structure of these animals, but the conformation of the upper antennæ seems at once to separate them from the foregoing genus. They are very minute, and from their mode of life on clayey bottoms or amongst mud, are not easy of detection. Though hitherto noticed in but few localities, they are probably more common than that circumstance might lead one to suppose.

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