600 MESSES. W. HILL AND A. J. JUKES-BROWNE [Nov. 1895, 



42. On the Occurrence of Radiolaria in Chalk. By W. Hill, Esq., 

 F.G.S., and A. J. Jukes-Browne, Esq., B.A., F.G.S. (Read 

 June 19th, 1895.) 



[Plate XXII.] 



Contents. 



Page 



I. Introductory Bemarks 600 



II. The Cbalk in which the Eadiolaria occur 601 



III. The Instability of the Radiolarian Skeleton 603 



IV. Description of the Forms met with 604 



I. Introductory Remarks. 



The rarity of radiolaria in rocks of Upper Cretaceous age is a re- 

 markable fact, and one for which it is not easy to account, seeing 

 that they are not uncommon in Lower Cretaceous rocks on the 

 Continent, and that they have been found in beds of Jurassic age in 

 France, Hanover, and Bavaria. One would moreover have expected 

 the physical conditions of the Upper Cretaceous period to have been 

 more favourable to their existence than those of Lower Cretaceous 

 or Jurassic times. It will be seen, from the descriptions which 

 follow, that one cause of their rarity is undoubtedly the facility 

 with which the siliceous skeletons of Polycystina disappear, when 

 they are embedded in rocks which contain a large proportion of 

 carbonate of lime. 



Our knowledge of Upper Cretaceous radiolaria comes chiefly from 

 Germany. In 1876 Prof. Zittel described six species, 1 occurring in 

 a Greensand of Senonian age in "Westphalia and Brunswick. 



In 1888 Dr. Riist described some well-preserved forms which he 

 had detected in phosphatic nodules from the Gault of Saxony, 

 Hanover, and Southern France, and in material filling the body- 

 chambers of ammonites from the same formation. 2 



He also described Dictyomitra anglica from flints in the Upper 

 Chalk of England {op. cit. p. 211 & pi. xxviii. fig. 16) and Dictyospyris 

 chlamydea, same occurrence {op. cit. p. 204). 



Besides those in the English flints mentioned by Riist there are 

 only two records of the occurrence of radiolaria in English Cre- 

 taceous deposits. Prof. W. J. Sollas, in 1873, describing the copro- 

 lites of the Cambridge Greensand, 3 which were afterwards shown 

 to have been derived from the Gault, says ' Polycystina and Xan- 

 tliidia occur in some sections,' but he does not figure or describe 

 them. Prof. Sollas has been kind enough to lend us the slides in 

 which these forms occur, but we find that they have well-preserved 

 tests, and differ both in shape and mode of preservation from those 

 which we have found in the Chalk. 



In 1883 Dr. Wallich recorded the occurrence of radiolaria in the 

 mealy material found inside hollow flints, mostly obtained from the 



1 Zeitschr. d. deutsch. geol. Gesellsch. vol. xxviii. 



2 * Palseontographica,' vol. xxxiv. (1888) pp. 181, 185. 



3 Quart. Journ. Greol. Soc. vol. xxix. p. 78. 



