MESOZOIC AND CAINOZOIC DINOFL AGELLATE CYSTS 215 



XI. FURTHER DINOFL AGELLATE CYSTS FROM THE LONDON CLAY 



By G. L. WILLIAMS & C. DOWNIE 



INTRODUCTION 



A number of species of dinoflagellate cysts from the London Clay have been 

 discussed in earlier chapters. However, there remains a large residue of species also 

 considered to be indigenous ; these are discussed in the ensuing pages. Further 

 work on assemblages from the lower part of the London Clay of the Sheppey area 

 and from the Thanet Sands (horizons considered to be Paleocene) is at present in 

 progress at Sheffield by Mr. A. Husain. 



Genus ADNATOSPHAERIDIUM nov. 



Diagnosis. Chorate cysts bearing tubular or solid intratabular processes varying 

 in number on a single plate. Processes united distally by interconnecting trabeculae. 

 Archeopyle apical. 



Type species. Adnatosphaeridium vittatum sp. nov. 



Discussion. Evitt (1961c) suggested that, in a taxonomic subdivision of Canno- 

 sphaeropsis into separate genera, three factors needed to be taken into consideration : 

 the type of archaeopyle ; the structure of the processes ; and the nature of the 

 interconnections between processes. All these points need consideration, but the 

 information at present available permits a subdivision into only two genera, recog- 

 nized by type of archaeopyle and the position of processes with regard to plates. 

 It is acknowledged that species now placed within Adnatosphaeridium represent 

 different morphological types and that further subdivision will be necessary. 

 Adnatosphaeridium includes species formerly placed in Cannosphaeropsis, which on 

 account of their intratabular processes, apical archaeopyle and spherical to ellip- 

 soidal shape, must be placed in a separate taxonomic category. When an archaeo- 

 pyle is present, species of Adnatosphaeridium can be readily distinguished from 

 those of Cannosphaeropsis. 



Adnatosphaeridium vittatum sp. nov. 

 PI. 24, figs. 3, 7 ; Text-fig. 56 



Derivation of name. Latin ; vittatus, decorated or bound with ribbons. 



Diagnosis. Ellipsoidal central body, with thin granular wall bearing processes 

 of two types : slender to broadly taeniate, greatly expanded distally, and hollow 

 open branched. Former type predominating. Adjacent processes often united 

 distally. Reflected tabulation ?', 6", 5'", ip and 1"". Archaeopyle apical. 



Holotype. B.M.(N.H.) slide ¥.519176(1). London Clay ; Sheppey, Kent, 

 sample Sh.3. 



Dimensions. Holotype : diameter of central body 37^, by 47[x, length of 

 processes up to 20\l. Observed range : diameter of central body, width 37-66^, 



