CRUSTACEANS
Of the ChydoridjE our list contains ten species distributed among seven genera, in the first five of which the head is not carinate above, in contrast with the other two in which it has a high thin keel. In Chydorus, Leach, from which as its oldest genus the family properly takes its name, the general form of the body in the female seen in profile is rounded or more or less globose. This character is further alluded to in the specific name of C. sphæricus (O. F. Mller), which is regarded as absolutely the commonest of all our British Cladocera. It was taken at Stoke Park and Burnham Beeches, and also in a pond on a common near the beeches. At the last of these localities there occurred with it llC. latusll, Sars. Among the distinctions between these two species the microscopist will be interested to observe that in C. sphæricus the length of the female is less than half a millimetre, whereas in C.latus it is equal to or a little greater than that half. The female therefore of the smaller species is less than a fiftieth of an inch long ; the male in each case is smaller than the female. Peracantha truncata (O. F. Müller) was taken both at Stoke Park and Burnham Beeches, Alonella rostrata (Koch) at the park, A. nana (Baird) at the beeches. In these two genera the fe- male seen in profile is not rounded, and while her rostrum in Peracantha is long and very acute, in Alonella it is not long and little acuminate. It must not however be dissembled that a beginner attempting to dis- criminate genera by help of one or two characters will find various stumbling blocks in his path. For instance, Professor Lilljeborg, from whose fine work on the Cladocera of Sweden these distinctions have been borrowed, says when describing the species A. nana that the general form of body in the female seen in profile is roundly subovate or sometimes almost rounded. Between the rounded Chydorus and the almost rounded Alonella the difference appeals to considerable acuteness of observation, when in one genus we are dealing with a species about a fiftieth, in the other with one about a hundredth, of an inch long. Again, we find Lilljeborg admitting that there was some reason for transferring Koch's rostrata to Alonella, though he himself prefers to place it in Alona, [1] but is not satisfied that it stands well in either genus.[2] When the seers are thus undecided their disciples are left in the lurch. Graptoleberis testudinaria (Fischer), taken at Burnham Beeches, Alona rectangula, Seers, from the same place, and A. quadrangularis (O. F. Müller) from Stoke Park, illustrate two genera which have the hind margin of the valves nearly equal to the greatest height of the valves, in contrast with the three preceding genera in which the hind margin is much less than that height. The altitude, it should be observed, is measured not from head to tail, but between the dorsal and ventral margins.
In Alona the rostral part of the head when seen in profile is narrowed and more or less acuminate. In Graptoleberis on the contrary this part from the same point of view is broad and rounded, so as to have a very characteristic appearance. Camptocercus rectirostris, Schöd-
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