176 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



VIII. THE GENUS HYSTRICHOKOLPOM A 



By G. L. WILLIAMS & C. DOWNIE 



INTRODUCTION 



The appearance of Hystrichokolpoma makes it one of the most striking of the 

 Tertiary dinoflagellate cysts. It occurs in small numbers throughout the London 

 Clay where its excellent preservation enables the tabulation to be determined 

 completely in many circumstances. Two previously described and one new species 

 are recorded. 



Genus HYSTRICHOKOLPOMA Klumpp 1953 : 388 



Emended diagnosis. Chorate cysts bearing two types of intratabular processes, 

 large types with expanded bases, and slender ones. Large processes have proximally 

 a quadrate cross section reflecting plate outline. Slender processes delimiting well 

 marked cingular and sulcal zone. Cingulum helicoid. Reflected tabulation of 4', 

 6", 6g, 5'", ip, 1"". Archeopyle apical tetratabular. 



Type species. Hystrichokolpoma cinctum Klumpp 1953. Eocene ; Germany. 



Discussion. Hystrichokolpoma is a genus with a spherical to ellipsoidal central 

 body possessing intratabular processes, radial symmetry and an apical archeopyle. 

 Each large process almost completely occupies a single plate proximally assuming 

 the outline of the plate leaving only a narrow border all round. Distally these 

 processes taper and may be open or closed. They are restricted to the apical, 

 precingular, postcingular, antapical and commonly anterior sulcal plates. The 

 antapical plate is easily recognizable, having a longer process than the others. The 

 slender processes are restricted to the cingular and sulcal zones. The number of 

 equatorial processes per plate is constant in an individual but can vary within the 

 species as now defined (personal communication from Dr. W. R. Evitt). The apical 

 plates are rarely found in position. Breakage along sutures readily occurs when 

 attempting to mount specimens of Hystrichokolpoma. Process variation in structure 

 and number is considerable and needs careful study. 



Hystrichokolpoma eisenacki sp. nov. 

 PI. 17, figs. 1-3 ; Text-fig. 46 



1954. Hystrichokolpoma cinctum Klumpp ; Eisenack : 64, pi. 10, figs. n-14. 



Derivation of name. After Prof. Alfred Eisenack, pioneer worker on fossil 

 dinoflagellates. 



Diagnosis. Ellipsoidal central body with wall composed of two closely appressed 

 layers — the smooth or granular endophragm and the thinner smooth periphragm. 

 Endophragm continuing uninterrupted beneath processes formed from periphragm. 



