vi INTRODUCTION. 



in Mr. Wright's list (op. cit., pp. 81 and 92). As in the Horstead material, these 

 Irish Entomostraca are similarly replaced by silica, and their state of preservation 

 is equally perfect. The list of species is given in the Appendix (pp. 55, 56). 



Colchester, Essex. — The Entomostraca from this locality were obtained by Mr. 

 John Brown, F.G.S.,^ from the Chalk passed through in boring an artesian well. 

 The species, as will be seen in the list in the Appendix (p. 56), are common 

 forms, and probably derived from the Chalk with flints. 



§ II. Clialk-roch.^ — This thin but well-marked band of hard, cream-coloured, 

 nodular limestone on the zone of Holaster planus, has also yielded a suite of Ento- 

 mostraca of which a list is given in the Appendix (pp. 57, 58). These have mostly 

 been obtained from outcrops of this rock at Dunstable and the railway-cutting 

 between Luton and New Millend (Bedfordshire), Chinnor (Oxfordshire), and West 

 Wycombe (Buckingham). 



§ III. Chalk-detritus. — Charing, Kent. — The nature of this deposit, from which 

 the large majority of species described in the ' Monograph Entomostraca Cret. 

 Form. Engl.,' 1849, were obtained, has already been referred to in that memoir 

 (p. 2). It is an extensive bed of soft, whitish clay, containing fragments of white 

 and grey Chalk, which has clearly been formed by the washing from the adjacent 

 Chalk hills, forming part of the North Downs in Kent. In this material a great 

 variety of Microzoa has been preserved, but of course it is not practicable to de- 

 termine the definite horizon from which each particular species has been derived. 

 This defect is to some extent compensated by the perfect state of preservation of 

 the specimens. A short notice and some figures of the commoner forms of the 

 Entomostraca in this " Detritus " were first given by Prof. Dr. W. C. Williamson,^ 

 who placed them in the Genus Cytherina. 



§ IV. " Greensand of Cambridge." — This bed of glauconitic marl, formerly 

 supposed to be on the horizon of the Upper Greensand, is now known to represent 

 the so-called Chloritic or Glauconitic Marl, and to be really the base of the Chalk- 

 marl, which rests here on an eroded surface of Gault. It contains numerous 

 Microzoa; and several species of Entomostraca from it, recorded by Prof. Sollas* 



• " Note on the Artesian "Well at Colchester ;" and " Eemarks on some of the Microscopic Fossils 

 from the Colchester Chalk," by John Brown, Esq., F.G.S., of Stanway, ' Ann. and Mag. Nat. Hist.' 

 ser. 2, vol. xii, 1853, pp. 240—242. 



2 For references to the character of this rock see the following: W. Whitaker, 'Quart, .lourn. 

 Geol. Soc.,' vol. xvii, p. 166 ; ib. vol. xxi, p. 398 ; ' Mem. Geol. Surv.,' vol. iv, 1872, p. 46. Jukes- 

 Browne, ' Geol. Mag.' dec. ii, vol. vii, p. 254 ; A. Geikie, ' Text-Book of Geology,' 2nd edit., pp. 821, 

 828 ; H. B. Woodward, ' Geology of England and Wales,' 2nd ed., pp. 403, 418 ; Judd, ' Quart. Journ. 

 Geol. Soc.,' vol. xl, p. 733 ; John Morrison, ' Transact. Hertfordshire Nat. Hist. Soc.,' vol. v, 1889, 

 pp. 199—202. 



3 'Mem. Lit. and Phil. Soc. Manchester,' 2nd ser., vol. viii, 1848, pp. 78—80, pi. iv, figs. 75—80. 



* ' Quart. Journ. Geol. Soc.,' vol. xxviii, 1872, p. 398-9. For other references to this bed see 

 T. G. Bonney, ' Proc. Geol. Assoc.,' iii (1873), p. 4, et seq. ; A. J. Jukes-Browne, ' Quart. Journ. Geol. 



