Chydorus, Camptocercus, Alona, Pleuroxus, and Peracantha, these, with the genus Acroperus, forming his family Lynceidæ. Here it is a little startling to observe that the family Lynceidæ is set up, but the genus Lynceus itself is shut out. Also three of the Müllerian species are not accounted for, namely, L. brachyurus, L. longirostris, and L. socors. Of these Baird refers the second to Bosmina, as a genus of the Daphniidæ; the third is perhaps unidentified; the first remains over for consideration.
In 1867 Norman and Brady, in their well-known paper on "The Families Bosminidæ, Macrothricidæ, and Lynceidæ," reassemble, under the title Lynceus, no less than eleven genera, no one of which, however, includes any of the three species omitted by Baird from the family Lynceidæ. The principle on which these two authors acted was that for generic distinction structural characters should be insisted on rather than variations of form. Though they do not make it very clear how the line is to be drawn between form and structure, there was something to be said for their opinion that generic subdivision in the family had been carried beyond the point which was justified by any thoroughness of contemporary knowledge. None the less they recognized the importance of the treatise by G.O. Sars on the 'Cladocera Ctenopoda,' 1865, and they contemplated the possibility that in the future some or all of the rejected names might properly be reinstated. This has come to pass. But authors have still been content to follow Baird in adopting a Cladoceran family Lynceidæ, in which no genus Lynceus was included, although of other generic names within that family the number has risen to at least eighteen. At length, however, in his 'Cladocera Sueciæ' (1900 or 1901), the veteran zoologist, Wilhelm Lilljeborg, raised a protest against this way of treating O.F. Müller's genus, and restored it in favour of Lynceus quadrangularis, Müller, which Baird had transferred in 1845 to a new genus Alona. But, as already explained, the action of Leach has affixed the title Lynceus to the species L. brachyurus. Accordingly, in any settlement of claims it is to that species that attention must first be given. It has nothing to do with Alona, for it is not one of the Cladocera at all. It is a phyllopod, and, as may be seen from the 'Fauna Norvegiæ' of G.O. Sars (vol. i. p. 116, 1896), in which it is fully described
