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 Cby- 

 dorus, 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. sphcerkus (O. F. M tiller), 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 

 C. /atus, Sars. Among the distinctions between these two species the 

 microscopist will be interested to observe that in C. sphcericus 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. Miiller) 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^ 

 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 rectangu/a, Sars, 

 from the same place, and A. quadrangular is (O. F. Miiller) 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. Gamptocercus recfirostris, Schod- 



1 For which however he adopts the preoccupied name Lynceus. 



' Cladocera Sueci<s, p. 48*. 



117 



