16 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



chorate cysts are represented by Cannosphaeropsis ; and M embranilamacia is typical 

 of the membranate group. 



A pterate group can also be recognized with Wanea as a representative, its main 

 characteristic being its pronounced equatorial outgrowth in the form of solid processes 

 linked distally or in mesh-like fashion. The marginate cysts form a group, typified 

 by Areoligera, consisting of chorate cysts whose outgrowths are characteristically 

 localized on the lateral margins, leaving the dorsal and more often the ventral 

 surfaces free of large outgrowths. 



Chorate cysts have a very long history, but they are more important in the Upper 

 Cretaceous and Tertiary than at any other time. 



(7) Cavate Cysts. 



In some forms already mentioned, small pericoels have been found between the 

 endophragm and periphragm. In the cavate group of cysts, the pericoel is a domi- 

 nant feature, so that the body consists of an inner body (capsule) formed by the 

 endophragm and an enclosing body, formed by the periphragm, often of quite a 

 different shape. 



The inner body is usually ellipsoidal, thick walled, with a smooth or granular 

 surface. It may have an archaeopyle but rarely shows any sign of tabulation. In 

 a few forms spinous projections bridge the gap between the endophragm and the 

 periphragm, but in many the cavity is more or less continuous and contact between 

 the two is either not apparent or occurs only at the mid-ventral line. The peri- 

 phragm is usually thinner and often smooth. It does however quite often have an 

 archaeopyle and may show a tabulation marked by sutural lines, or by intratabular 

 spine pallisades as in Wetzeliella. The overall shape is an important feature in these 

 cysts, especially the number and positions of the horns on the periphragm. 



A number of sub-groups are recognized depending on the degree of continuity of 

 the pericoel. In typical cavate cysts like Deflandrea phosphoritica there is only one 

 large pericoel extending over nearly all the body. In bicavate cysts there is a wide 

 zone of contact round the equator which divides the pericoel into an apical and 

 antapical part ; Triblastula is typical of this group. Stephodinmm represents a 

 small group (the pterocavate cysts) with a pronounced equatorial pericoel. Small 

 pericoels may occur in other groups such as the apical pericoels in some proximate 

 cysts, the gonal pericoels in some Hystrichosphaera species. 



(8) Other Cyst Groups. 



In addition to the three main groups discussed so far one must recognize the 

 existence of groups of fossil dinoflagellates with calcareous tests and with siliceous 

 tests. Neither of these groups is being considered in this paper, although most 

 appear to be non-motile and probably encysted. 



Classification. Eisenack (1964) published a classification of fossil dinoflagel- 

 lates which is by far the most comprehensive classification yet attempted. In this 

 paper he adopts a botanical system, which is welcomed. However, his classification 



