MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 29 



in Gonyaulacysta (Deflandre 1964). In 1963 he erected the genus Achmosphaera to 

 accommodate species possessing a precingular archaeopyle and processes of the same 

 form and distribution as found in the genus Hystrichosphaera, but lacking sutural 

 crests or membranes. 



Genus HYSTRICHOSPHAERA O. Wetzel 1933 : 33 

 1937. Hystrichosphaera O. Wetzel ; Deflandre : 61. 



Emended diagnosis. Chorate to proximo-chorate cysts possessing a sub- 

 spherical or ovoidal central body with a clearly defined reflected tabulation of 

 3-4', 6", 6c, 5'", o-ip, 1"", plate 6" being generally reduced and triangular. Wall of 

 central body composed of two layers, an inner endophragm and an outer periphragm. 

 Cingulum always disposed in a laevo-rotatory spiral. Plate boundaries indicated by 

 variably developed sutural crests or membranes, and gonal and sutural processes. 

 Processes open or closed, solid or hollow, simple or branching. Length of processes 

 variable, sometimes not extending beyond sutural crests, apical pole often marked 

 by an elongate process. Archaeopyle precingular, formed by loss of reflected plate 

 3"- 



Type species. Xanthidium ramosa Ehrenberg 1838. Upper Cretaceous (Seno- 

 nian) ; Germany. 



Remarks. The generic diagnosis is emended to include reference to the reflected 

 tabulation and to the presence of sutural processes in many species attributable to 

 this genus. 



The Hystrichosphaera furcata-ramosa complex 



It is difficult to distinguish between H. furcata (Ehrenberg) and H. ramosa (Ehren- 

 berg). The original drawings of Ehrenberg are inadequate for the present refined 

 morphological studies and there is no description accompanying the figures of the 

 types. The types for both species have either been lost or have not been re-examined 

 recently, and many varied interpretations of the species have been made by later 

 workers. 



Ehrenberg (1838) figured a number of specimens as Xanthidium furcatum and 

 Xanthidium ramosum without description or holotypes. His figures show that the 

 main difference between the two species is the form of the extremities of the processes. 

 X. furcatum has predominantly bifurcate processes ; only one of the figures (pi. 1, 

 fig. 14a) shows trifurcate processes and then these do not predominate. X. ramosum 

 has predominantly trifurcate processes and branching is shown to occur occasionally 

 from a medial position on the processes. Branching is absent from Ehrenberg's 

 figures of X. furcatum. The names given to the species confirm that Ehrenberg 

 distinguished them on the type of process present. 



White (1842) published the first account of both species. X. furcatum he described 

 as having numerous, regularly arranged processes which gradually taper distally ; 



