64 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Hystrichosphaeridium readei sp. nov. 

 PI. 6, fig. 3 



?I940. Hystrichosphaeridium salpingophorum (Deflandre) ; Lejeune-Carpentier : 219, text- 

 fig- 7- 

 ?i940. Hystrichosphaeridium tubiferum (Ehrenberg) ; Lejeune-Carpentier : 218, text-fig. 5. 



Derivation of name. Named in honour of the Reverend J. B. Reade who was 

 the first to describe and figure (1839) hystrichospheres from English flints of the 

 Upper Cretaceous age. 



Diagnosis. Spherical to subspherical central body composed of smooth endo- 

 phragm and periphragm. Processes, composed of the latter, slightly fibrous, 

 cylindrical and open. Larger processes with two thickenings of periphragm extend- 

 ing along their length over surface of central body and joining up with similar 

 thickenings from neighbouring processes. Processes varying in width and expanding 

 distally, terminating in fairly complicated aculeate or secate margin. Apical 

 archaeopyle usually present. 



Holotype. Geol. Surv. Colin, slide PF.303o(2). Lower Chalk, H.M. Geological 

 Survey Borehole, Fetcham Mill, Surrey, at 690 feet depth. Upper Cretaceous 

 (Cenomanian). 



Dimensions. Holotype : diameter of central body 41 by 45[x, length of processes 

 23-2951., number of processes 24. Range : diameter of central body 3i-54fi., length 

 of processes 20-35^. Number of specimens measured, 8. 



Description. The most characteristic feature of this species is the presence of 

 thickenings or ribs in the periphragm. These ribs are close together near the distal 

 end of the process and gradually diverge proximally, before separating completely 

 on the surface of the central body, each rib passing to a different neigbouring process. 

 Thus triangular and polygonal networks are seen on the surface of the central body. 

 The sulcal processes are usually noticeably finer than the others and are very occa- 

 sionally joined by septa, as seen in the holotype. The septa are probably modified 

 ribs. An apical archaeopyle is usually present. The number of processes in the 

 specimens observed varied between 23 and 27. The reflected tabulation is difficult 

 to determine but it is probably comparable to H. tubiferum i.e. 4', 6", 6c, 5'", ip, 

 1"" and xs. 



H. readei sp. nov. occurs in the Upper Cenomanian at Fetcham Mill, Surrey and 

 has not been recorded lower in the succession at this locality. 



Remarks. The Cenomanian species strongly resembles and is undoubtedly 

 related to, H. costatum from the Upper Jurassic. The only apparent difference 

 between the species is in the extremities of the processes. In H. costatum the 

 extremities are simply denticulate with a small number of spines, whereas in H. 

 readei the extremities are considerably more complex. 



