Vol. 51.] ON THE OCCURRENCE OP RADIOLARIA IN CHALK. 605 



follow as nearly as possible the system of Prof. Haeckel in his 

 report on the radiolarians of the ' Challenger ' Expedition, also 

 following Dr. Hinde in his recent description of radiolarians in the 

 Lower Palaeozoic Bocks of the South of Scotland, Ann. & Mag. Nat. 

 Hist. ser. 6, vol. vi. (1890) p. 40. 



It will he understood that the forms described below are those 

 seen in thin sections of Chalk, and therefore there is the initial 

 difficulty in our being unable to say positively whether the tests are 

 spherical or discoid. 



Class RADIOLARIA, Miiller. 

 Subclass SPUMELLARIA, Ehrenberg. 



Order Sph^brellaria, Haeckel. 



Suborder Sphseroiclea, Haeckel. 



Family 1. Ltosph^rida, Haeckel. 



Sphseroidea without radial spines on the surface of the spherical 

 shell, living solitary (not associated in colonies) : ' Chall.' Kept. pt. i. 

 p. 59. 



Genus Oenosphcera ? (PI. XXII. fig. 1.) 



A simple well-defined circular outline, usually filled with clear 

 granular calcite, but sometimes with denser material (the finer 

 amorphous matrix of the chalk). There is no evidence of radiate 

 spines or of a medullary shell. 



Diameter *22 to "35 millim. 



It occurs in every locality where we have found radiolarians. 



Genus Spongoiilegma or Spongodictyon ? (PI. XXII. figs. 2 & 3.) 



Another form which we refer to this family is in section a simple, 

 well-defined circle without spines. Within the outline the infilling 

 material is usually dense, and suggests a radiolarian with ' sponge- 

 like ' test. In several cases there is a well-marked centre of clearer 

 crystalline material occupying the position of a medullary shell. 



Diameter *25 to - 45 mm. 



Localities: Hitchin, Luton, Dover. Horizon: Melbourn Rock. 



Family 2. AsTRosPHiERiDA, Haeckel. 



Sphaeroidea with numerous (8 to 12 or more, commonly between 20 

 and 60) radial spines on the surface of the spherical shell ; living 

 solitary (not associated in colonies) : ' Chall.' Eept. pt. i. p. 206. 



Forms which we refer to this family are circles with radiating 

 spines, the number of which does not in any case exceed six ; 

 these appear to proceed from the surface of the test, and to be ir- 

 regularly disposed over its surface, not arising in pairs or with any 

 degree of symmetry with regard to each other. The spines are not 

 bristle-like, but have considerable width and length ; none seem 

 quite perfect, nor can their junction with the test be seen in every 

 ■case. 



