MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 133 



Genus HESLERTONIA nov. 



Derivation of name. Based on the name West Heslerton, Yorkshire, from 

 which the type species was first described. 



Diagnosis. Chorate dinoflagellate cysts, spherical, ellipsoidal, ovoidal or poly- 

 hedral, with high sutural crests outlining the tabulation 3-4', o-ia, 6", 6g, 6'", ip, 

 0-1 p. v., 1"". Cingulum strongly or weakly helicoid ; sulcus generally but not 

 constantly extending on to epitract. Apical and antapical horns absent. Sutures 

 perforate or imperforate ; their distal edges smooth or denticulate. Crest height 

 exceeds \ of shell width. Surface smooth, granular, punctate, nodose or reticulate. 

 Precingular archaeopyle formed by loss of plate 3" ; archaeopyle sometimes absent. 



Type species. Gonyaulax heslertonense Neale & Sarjeant 1962. Lower Cretace- 

 ous (Hauterivian) ; England. 



Remarks. This genus is a chorate equivalent of Gonyaulacysta, characterized by 

 its disproportionately high crests. 



Heslertonia heslertonensis (Neale & Sarjeant) 

 1962. Gonyaulax heslertonense Neale & Sarjeant : 440, pi. 19, fig. 5, pi. 20, fig. 5 ; text-fig. 1. 



Remarks. Forms closely comparable to this species have been figured and 

 described from the Middle Cretaceous of Australia as Cymatiosphaera striata Eisenack 

 & Cookson (i960 : 9, pi. 3, figs. 10, 11). Both forms have in common high, striate 

 crests : the figures suggest that the Australian species also has a cingulum and a 

 determinable tabulation. A full restudy of the Australian specimens appears neces- 

 sary to determine whether H. heslertonense and C. striata are congeneric or conspecific ; 

 should the latter prove to be the case, the earlier name striata would have priority. 



Genus LEPTODINIUM Element i960 : 45 



Emended diagnosis. Proximate dinoflagellate cysts, spherical, ovoidal, ellip- 

 soidal or polyhedral, with the tabulation 4', 6", 5-?6g, 5'", ip, 1"". Cingulum 

 strongly or weakly helicoid, sulcus generally but not constantly extending on to 

 epitract, undivided or subdivided into a pattern of ventral plates. Sutures between 

 plates in form of low ridges, or bearing crests of varied form (smooth, denticulate or 

 spinous ; perforate or imperforate). Height of crests always less than \ of shell 

 width. Surface of shell smooth, granular or punctate. Precingular archaeopyle 

 formed by loss of plate 3" ; not all individuals may show an archaeopyle. 



Type species. Leptodinium subtile Klement i960. Upper Jurassic (Oxfordian 

 to Lower Kimmeridgian) ; Germany. 



