io8 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Fossil forms, from the Upper Jurassic (Oxfordian) of France, were first attributed 

 to this genus by Deflandre (1938). A tabulation pattern corresponding exactly to 

 that of Gonyaidax was found to be exhibited by four species, which were named 

 G. jurassica, G. cladophora, G. eisenacki, and G. pachyderma. A feature noted was 

 the frequent absence of precingular plate 3", but the significance of this was not 

 immediately recognized. A subsequent study by Deflandre of French Kimmeridgian 

 sediments yielded further new species with the characteristic tabulation : and 

 subsequent studies by various authors showed that fossils with a tabulation of this 

 type were present from the Upper Triassic to the Oligocene, attaining greatest 

 abundance in the Upper Jurassic and Lower Cretaceous. 



A number of other genera with a corresponding or closely similar tabulation have 

 since been proposed. The genus Ctenidodinium was proposed by Deflandre (1938) 

 for Upper Jurassic forms having a strong, denticulate crest on the posterior margin 

 of the cingulum, but with only a low ridge on its anterior margin. These forms 

 split by schism along the line of the cingulum. Klement (i960) demonstrated that 

 the tabulation of these forms corresponded to that of Gonyaidax and proposed 

 abandonment of the name Ctenidodinium. 



The genus Lithodinia was formulated even earlier (Eisenack 1935) for forms with a 

 partially silicified shell from the Middle Jurassic (Dogger) of the Baltic. Subse- 

 quently Eisenack (1961) stated that he considered Lithodinia to be congeneric with 

 Gonyaidax. The author was courteously allowed to examine the genotype of 

 Lithodinia during a visit to Tubingen in 1962 ; the tabulation certainly corresponds 

 closely to the Gonyaidax pattern. 



The genus Microdinium was proposed by Cookson & Eisenack (i960) for forms 

 having a tabulation pattern as follows ; 1', 6", ?6c, 6'", ip, 1"", the shell opening by 

 loss of the apical plate. This tabulation accords with that specified by Kofoid (191 1) 

 and the genus is thus technically invalid ; however, the majority of fossil species 

 attributed to Gonyaulax have three to four apical plates. 



The genus Hystrichosphaera, which has a Gonyaulax-type tabulation, is treated 

 with in an earlier section. There are in addition six other described fossil genera 

 showing a tabulation resembling, but not exactly corresponding to, that of Gonyaulax : 



Cryptarchaeodinium Deflandre 1939, described from the French Upper Jurassic 

 (Kimmeridgian), has the tabulation 4', 6", 7'", i-?2p, ?i"". This differs from that of 

 Gonyaulax in the presence of an extra postcingular plate and in the presence of three 

 plates (not yet clearly designated) in the antapical region. 



Eisenackia Deflandre & Cookson 1955, described from the Lower Tertairy of 

 Australia, has the tabulation ?3', 6", ? 6 c, 6'", i-2p, 1"". It differs from Gonyaulax, 

 and corresponds to Cryptarchaeodinium, in having three plates in the antapical 

 region, but differs from the latter genus in the possession of only two or three apical 

 plates. 



Leptodinium Klement i960, described from the Upper Jurassic of Australia, has 

 the tabulation 4', 6", ?6c, 5'", ip, 1"". It differs from Gonyaulax in the lack of any 

 anterior intercalary plate and the presence of one fewer postcingular plate. 



