MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 153 



Type species. Gonyaulax rhaetica Sarjeant, 1963a. Upper Triassic (Rhaetic) ; 

 England. 



Remarks. This genus is created to accommodate the earliest known species 

 having a tabulation of Gonyaulax type. The shape is unlike that of any other known 

 fossil species having such a tabulation ; the plate boundaries are so poorly marked as 

 to make it seem probable that any descendants would be non-tabulate. For these 

 reasons, it is considered that a relationship with the species attributed to the genus 

 Dichadogonyaulax gen. no v. is unlikely. Later genera having a spindle-shaped 

 outline (Kalyptea ; Netrelytron) appear to consistently form precingular archaeopyles. 



OTHER SPECIES 



The following species also accords with the diagnosis of this genus and is here 

 included in Rhaetogonyaulax accordingly : Rhaetogonyaulax chaloneri (Sarjeant 

 1963a). Upper Triassic (Rhaetic) ; England. 



Genus DICHADOGONYAULAX nov. 



Derivation of name. Greek, dichados, half : refers to the almost median 

 schism of the shell, so that typically half shells are encountered, and to the Gonyaulax 

 — type tabulation. 



Diagnosis. Proximate dinoflagellate cysts, spheroidal, ovoidal, ellipsoidal or 

 polyhedral, having the tabulation 3-5', o-?ia, 6", 5 6'", ip, 1"" : sulcus divided into 

 plates or undivided, ventral region may show division into additional small plates. 

 Cingulum strongly or weakly spiral, laevorotatory. Apical horn may be present ; 

 median and antapical horns lacking. Sutures in form of low ridges bearing crests of 

 varied form (smooth, denticulate or spinous ; perforate or imperforate) ; or marked 

 by lines of spines. Archaeopyle (where present) epitractal, by schism of shell 

 immediately anterior to cingulum. 



Type species. Gonyaulax culmula Norris 1965. Upper Jurassic (Portlandian) ; 

 England. 



Remarks. Norris (1965) has described a group of species from the Portlandian 

 which have in common their general morphology and mode of archaeopyle forma- 

 tion, but whose tabulation accords sometimes to the Gonyaulax pattern, sometimes 

 to that of Leptodinium. The tabulation of the type species, which lacks an anterior 

 intercalary plate but possesses six postcingular plates, is indeed intermediate in 

 character. On this basis, it is considered that common characters outweigh the small 

 tabulation differences and that the diagnosis should embrace all forms with epitractal 

 archaeopyles and generally similar morphology. 



other species 

 The following species also accord with the diagnosis of this genus : 



Dichadogonyaulax pannea (Norris 1965) comb. nov. Upper Jurassic (Upper 

 Kimmeridgian-Portlandian) ; England. 



D. schizoblata (Norris 1965) comb. nov. Upper Jurassic (Upper Portlandian) ; 

 England. 



