A HISTORY OF ESSEX to discriminate all these by a few luminous strokes of the pen, but no more can be attempted here than to indicate the difficulties of such an undertaking. Always, everywhere, and by all men, the common water flea has been known as D. pu/ex, de Geer. Why then does M. Jules Richard in his important Revision des Cladoceres write it down as D. pu/ex, Leydig ? His reason is simple: that Leydig in 1860 was the first to give a really satisfactory description of a species under that name, earlier writers having left it uncertain what particular species or what jumble of species may have been intended by what they chose or chanced to call D.pulex. Only Zenker, he says, in 1851 had already noted ' the long ciliated abdominal prolongation of the male,' which is highly characteristic for that sex, the females being recognizable by the very small first antennas, the concave ventral border of the head, and the general shape. 1 Dr. G. S. Brady, citing both de Geer and Leydig, describes and figures both sexes of this species, the male from a pond at Whipscross Road, Essex, whence Mr. Scourfield had supplied him with specimens. 2 For D. magna a characteristic feature is found, not in the abdomen, but in the post-abdomen. This in the female has the dentate parts of its dorsal margin separated by a deep sinus. In the male it ' bears in front of the terminal unguis a finger-like lobe,' in allusion to which Dr. Brady calls the new genus in which he places it Dactylura, finger-tail. 3 Of his 'Daphnia(1} ga/eata, Sars,' Mr. Scourfield says : ' By comparison with specimens of D. ga/eata kindly sent to me by Prof. Sars, I have been able to see that our Epping Forest form is not only not a typical representative of the species, but that it may even be quite distinct. As I cannot decide, however, to which of the other hyaline species it belongs, I have preferred to continue to refer it doubt- fully to D. ga/eata. The typical D, ga/eata has been recorded as British by Prof. Brady.' According to Brady's figures this ' helmeted ' form is very variable and sometimes of rather comical aspect. Upon his other doubtful species, Mr. Scourfield remarks : ' This form, which I have recorded in the paper on the Entomostraca of Wanstead Park [Journ. Quekett Micro. C/ut>, 1893] as D. cucu//ata, is almost exactly similar to the foregoing species, but is without the eye-spot. It is certainly not a characteristic representative of D. cucu//ata, but, on the other hand, it cannot with greater certainty be referred to any other species.' He suggests its possible identity with ' D. kablbergensis, Brady' (1898), but this reference is a little inexact, since Brady writes * Hyalodaphnia kabl- bergensis (Schcedler),' though in regard to Schodler's Hyalodaphnia he agrees with Richard that it scarcely differs from Dapbnia except by the absence of an eye-spot. In both of these genera the first antennae of the female are immovable ; in Ceriodapbnia, Dana, they are movable, and of this genus Mr. Scourfield finds five species in Essex, C. mega/ops, Sars, C. rotunda (Straus), C. reticulata (Jurine), C. quadrangula (O. F. Miiller) 1 Annales des Sdencei Naturellet, ser. 8, vol. ii. p. 235 (1896). 2 Nat. Hiit. Trans. 'Northumberland, etc., vol. xiii. pt. 2, p. 223 (1898). s Loc. cit. p. 240. 212
