MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 157 



VII. FOSSIL DINOFLAGELLATE CYSTS ATTRIBUTED TO B A LT I SPH AERI DI U M 



By R. J. DAVEY, C. DOWNIE, 

 W. A. S. SARJEANT & G. L. WILLIAMS 



INTRODUCTION 



The genus Baltisphaeridium was proposed by Eisenack (1958 : 398) to accommo- 

 date species of fossil microplankton having spherical to oval, nontabulate shells 

 bearing simple or branching appendages, consistently closed distally. The type 

 species selected was the Silurian species B. longispinosum, having a size range of 40 to 

 75[x. Eisenack did not compare his new genus with the existing genus Micrhy- 

 stridium Deflandre 1937 (defined as having a shell diameter inferior to 20[i.) : subse- 

 quent workers, however, assumed a separation between the two genera on the basis 

 of the size restriction of Micrhystridium. 



Staplin (196 1 : 408) proposed the redefinition of Micrhystridium by restricting it to 

 forms having appendages closed distally and by removing the size restriction ; this 

 redefinition made Baltisphaeridium into a junior synonym of Micrhystridium. 

 This proposal was attacked by Eisenack (1962 : 96) and Downie & Sarjeant (1963 : 

 83-84) ; the latter authors, while recognizing the arbitary nature of the upper size 

 limit of Micrhystridium, nevertheless considered that this genus expressed a natural 

 morphological grouping distinct from Baltisphaeridium. They proceeded to give 

 an emended diagnosis for Baltisphaeridium, as follows : 



" Hystrichospheres with spherical to oval shells not divided into fields or plates, 

 bearing + numerous processes, simple, branching or ramifying, hollow to solid, 

 always with closed tips. The processes are not connected together distally and no 

 outer shell, complete or incomplete, is present : the processes are most often of a 

 single basic type, but processes of two or more types may be present. Mean and 

 modal diameter of the shell greater than 20\l." 



At this date, separation of acritarchs from dinoflagellate cysts had not been made. 

 Within the genus Baltisphaeridium, there were placed forms having pylomes ; 

 forms with archaeopyles, variously situated ; and forms having no openings of any 

 kind. Thus, within a single genus, there were classed together both species of 

 demonstrable dinoflagellate affinity and morphologically similar forms of unproven 

 and perhaps quite different affinity. 



An attempt is here made to remedy this confused situation. Species having 

 spheroidal to ovoidal shells with apical archaeopyles, with the processes arranged so 

 as to give a reflected tabulation 4', oa, 6", 6c, 6'", ip, 3"", are placed into the new 

 genus Surculosphaeridium. Species having a spheroidal to ovoidal shell with an apical 

 archaeopyle, but having numerous processes and an undetermined or intermediate 

 tabulation, are placed in a second new genus, Cleistosphaeridium. Species having an 

 elongate ovoidal to ellipsoidal shell, with an apical archaeopyle and processes arrang- 

 ed into distinct rows, are placed in a third new genus Prolixosphaeridium. Species 

 having a precingular archaeopyle are placed in a fourth new genus Exochosphaeridium. 

 The residue of species, either with a circular pylome (such as the type species, B. 

 longispinosum) or with no observed opening, are considered to be acritarchs and left 



