CRUSTACEANS only three swimming setae in Ilyocryptus, Sars, but four in Macrothrix, Baird, and five in Latbonura, Lilljeborg. 81 Further, Ilyocryptus has six pairs of feet, with the last pair rudimentary, Macrothrix has only five pairs, with the last of them not quite rudimentary, and Latbonura also has only five pairs, but here it is not only the fifth pair that has dwindled, for the fourth also is so small that its existence has often been overlooked. Ilyocryptus sordidus (Lievin) owes its title of ' sordid mud-burrower ' to its habits. Lilljeborg states that it occurs pretty frequently in lakes and slow-flowing rivers, where it lives exclusively on the muddy floor. Back downward, the shell-valves open, the feet in movement, it lies generally at rest on the bottom ; its second antennae are used only for crawling and burrowing. 8 * In spite of its lethargic nature, however, it has spread itself about Europe and Africa, and is known from Australia and North America. Macrothrix laticornis (Jurine), which occurs also in Mr. Scourfield's list, owes its specific name to the character of the first antennae in the female. These, instead of narrowing towards the apex, are in this species dilated. The generic name, meaning long-haired, refers in fact to a single hair, or rather seta. In this genus the three-jointed branch of the second antennae, which Lilljeborg calls the inner, and Baird the anterior, has five natatory setae, three on the apical joint, and one seta apiece on the other two. This seta, or filament, on the first joint is described by Baird as ' much longer than any of the others.' 2S The superiority in length does not appear to be constant, but the seta has other distinctive features which still make it of importance. Latbonura rectirostris (O. F. M.) has the hind-body small and thick, in the living animal generally concealed between the feet, a * tail-hiding ' propensity of which the generic name is significant. The pair of caudal setae in this species are very long. The ephippial females are said to carry as many as from five to seven winter-eggs in the detachable part of their organism, called the ephippium from its resemblance to a saddle. 24 Our remaining species of the Cladocera are all included in the extensive family of the Chydoridae, Eurycercus lamellatus (O.F.M.) being distinguished from the rest by having to the front of the intestine two short coeca of which the others are devoid. Its hind-body is very large and broad, strongly compressed, so that the hinder or upper part is thin and lamellar, and this is fringed with a single row of little spines or teeth, amounting in old specimens to more than a hundred. It has been taken both by Mr. Garnar and Mr. Scourfield. There are still eight genera to be discriminated. In only two of them, Camptocercus, Baird, and Acroperus, Baird, is the head carinate above. Of the other six, two, Alona, Sars, and Leydigia, Kurz, have the free hind margins of the valves little lower than the rest of their height, while in the remaining four they are much lower. In Chydorus, Leach, the body of the female is rounded in profile and more or less globose. In the other three genera it is not rounded, and of these Alonella, Sars, has the rostrum neither long nor very acute, in contrast to Peracantha, Baird, and Pleuroxus, Baird, in which the rostral characters are just the opposite. The female Camptocercus has a long slender tail, spined on the upper margin, whereas in Acroperus this tail is of medium length and breadth, and on its upper margin not spined. Camptocercus rectirostris, Schodler, is described as 11 Cladocera Sueciae, 210. " Ibid. 331. n British Entomostraca (Ray Soc.), 103 (1850). " Cladocera Sueciae, 360. I0 3
