1.-4 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Diagnosis. A Gonyaulacysta with ovoidal to broadly ellipsoidal theca and strong, 

 tapering apical horn accounting for about one-sixth to one-seventh of overall length. 

 Tabulation 4', ia, 6", 6c, 6'", ip, 1"". Crests delimiting plates represented by low 

 ridges, minutely serrate ; those bordering cingulum higher. Cingulum strongly 

 spiral, of moderate breadth ; sulcus sunken, broad, with only short epitractal section. 

 Surface of shell densely granular ; granulation less pronounced on sulcus. 



Holotype. B.M.(N.H.) slide ¥.51730(2). Speeton Clay, Shell West Heslerton 

 Borehole No. 1, West Heslerton, Yorks., at 19-25 metres depth. Lower Cretaceous 

 (Upper Barremian). 



Dimensions. Holotype : overall length ioi[x, breadth 64^ (minimum) ; shell 

 length 841X, breadth 56(1 (minimum) ; length of horn, i6[x ; width of cingulum 

 c.4-5fx. Range of dimensions : overall length c.95-i05[x, breadth c. 55-65(1. 



Description. This is a relatively infrequent species, some 6 specimens only 

 having been studied. The holotype was the best oriented for study, but is somewhat 

 crushed in at right, so that breadth measurements stated are minima and the right- 

 hand tabulation is possibly inaccurate, albeit confirmed in some measure by study of 

 other specimens. • 



There are four apical plates ; the apical horn is tipped by plate 4', its flanks being 

 formed by the other three plates. Plate 1' is especially elongate, extending down 

 almost two-thirds of the epitract. Six precingular plates are present, plate 6'' being 

 reduced to accommodate a very large anterior intercalary plate. Six post-cingular 

 plates are present, with plate 1'" reduced and very elongate ; an elongate posterior 

 intercalary plate lies between it and the antapex. A single, rather quadrate plate 

 occupies the antapex. 



The cingulum forms a strong laevorotatory spiral such that its two ends differ in 

 antero-posterior position by twice its width. It is bordered by strong ridges. The 

 cingular plates are poorly defined, but appear to be six in number. The sulcus is 

 broad and relatively short, extending to the antapex. 



The shell surface is densely granular. The crests are low but generally readily 

 perceptible : they are finely but irregularly serrate. 



A precingular pylome, formed by loss of plate 3", is present in all specimens seen. 



Remarks. Gonyaulacysta aichmetes sp. no v. is of a general form similar to several 

 other Cretaceous species, but is distinguished by shape, crest character and detail of 

 the tabulation. The most comparable species is undoubtedly Gonyaulacysta 

 apionis (Cookson & Eisenack 1958) from the Albian of South Australia. However, 

 this latter species has a more ellipsoidal shape, smooth crests and four pronounced 

 projections bordering the antapical plate ; its tabulation has not been fully specified, 

 but, from the figures given (Cookson & Eisenack 1958, text-figs. 3, 4) plate 1 appears 

 to be less elongate, there appears o be no anterior intercalary plate, and the posterior 

 ventral structure appears different. The " lid " on the apical horn of G. apionis 

 is probably simply a small apical plate edged by crests, such as is present in G. 

 aichmetes. 



