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. 



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