MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



115 



Description. Shell spheroidal, slightly broader than long, with epitract sur- 

 mounted by a dome-like outgrowth of the periphragm enclosing an apical pericoel. 

 A slender, furcate horn arises from the apex. The dome-like structure arises from 

 the confluent crests of the plates on the apical portion of the epitract ; position of 

 three -? four apical plates and an anterior intercalary plate are indicated by dimples 

 in this structure. Six precingular plates are present, plate 6" being of reduced size. 

 Six postcingular plates are likewise present, with plate 1'" slightly reduced to accom- 

 modate the posterior intercalary plate. A polygonal plate occupies the antapex ; 

 from its centre arises a short, blunt antapical process. Slender spines at the angles 

 sustain the crests surrounding the antapex ; these crests are not connected to the 

 antapical process. 



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

 in antero-posterior position by more than twice its width. It is divided into six 

 cingular plates. The sulcus is sinuous rather than sigmoidal : a median section along 

 its line would touch the two ends of the cingulum but would not cut them. It is of 

 moderate breadth in its epitractal portion, but widens as it approaches the antapex. 



The crests are high and delicate, with denticulate edges ; they are irregularly 

 perforate, the perforations being so fine as to be almost imperceptible at high 

 magnifications (x 1,000). The surface bears an irregular, sparse scatter of coarse 

 tubercles, but is otherwise smooth. 



No archaeopyle has been observed. 



Fig. 25. Gonyaulacysta axicerastes sp. nov. Tabulation, 

 right, dorsal view. x c. 1000. 



Left, ventral view 



