M 



MESOZOIC AND CAINOZOIC DINOFL AGELLATE CYSTS 



breakage along and within the cingulum. Apical archaeopyles are called haplo- 

 tabular when they consist of a single plate and tetratabular when they have four. 



(5) Proximate Cysts. 



The proximate cysts are an important group, which have always been recognized 

 as dinoflagellates because their resemblance to modern forms is very close. Never- 

 theless, although all the features of tabulation may easily be determined on good 

 specimens, they are not motile thecae, but cysts. The terms epitheca and hypo- 

 theca used for motile stages, are therefore inappropriate. It is here proposed that 

 their cyst equivalents should be termed epitract and hypotract, names suggested by 

 G. L. Williams (thesis, 1964). 



Contraction in the formation of the proximate cyst is generally to about \ or § of 

 the original volume; this can roughly be gauged by the height of the periphragm 



Radius of endocoel 



spines or lists. The ratio 



= 0-8 constitutes a rough limit to the 



Radius overall 

 proximate cysts. 



The periphragm often forms extensions which bear a close relationship to the 

 presumed tabulation of the motile stage theca. These extensions are nearly always 

 sutural (i.e. reflecting the site of the sutures on the motile stage) and most often 

 consist of continuous crests or lists, often with serrate or denticulate edges. Less 

 commonly, lines of small solid spines or tubercles mark the reflected sutures. 



Subdivision of the proximate group of cysts is based on a number of features among 

 which the reflected tabulation, the overall shape of the test (i.e. elongation, number of 

 horns) and the nature of the archaeopyle are important. 



Typical of the proximate cysts is the genus Gonyaulacysta. The group as a whole 

 is more characteristic of Upper Jurassic-Lower Cretaceous than of later strata. 







c aulit lorate 



bulbous 

 jvexate capitate 



foliate 



bifid 



I r T n n 



oblate branched trifurcate 



digitate bifurcate entire 



fenestrate 



|****T h(wJ 



aculeate denticulate patulate orthogonal 



secate recurved dirigate 



^f= 



Fig. 4. Illustrations of the terms used to describe the various kinds of 

 distal termination of the processes. 



