5-' MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Remarks. The Hauterivian examples strongly resemble those forms illustrated 

 by Eisenack (1958) from the Aptian of Germany. In both of Eisenack's photographs 

 (pi. 26, figs. 7 and 8) the views are either apical or antapical, a rather misleading 

 orientation, the archaeopyle being seen in profile on the top side of these figures. 

 The antapical view is shown in one of the specimens figured here, pi. 3, fig. 7, the 

 archaeopyle being to the north-east. The processes of A . neptuni are very character- 

 istic and it should be noted that they are not distally bifurcate as is usual in this 

 genus and in Hystrichosphaera. 



OTHER S PECIES 



Achomosphaera hyperacantha (Deflandre & Cookson). This species, Hystricho- 

 sphaera hyperacantha Deflandre & Cookson 1955, which possesses very faint or 

 invisible plate outlines is here considered to belong to the genus Achomosphaera 

 Evitt and is renamed accordingly. Miocene ; Australia. Achomosphaera triangu- 

 lata (Gerlach 1961 : 194, pi. 29, fig. 1) is here transferred to Achomosphaera on the 

 basis of the possession of a precingular archaeopyle and the arrangement of the 

 processes. Miocene ; Germany. 



CONCLUSIONS 



The characteristics and known stratigraphical distribution of the species and 

 varieties of the genus Hystrichosphaera are summarized in the accompanying table 

 (Table 2). The varieties of H. ramosa, as has been previously pointed out, are 

 intergrading and, as one would expect, no clear stratigraphical picture emerges. 

 The more distinctive species of this genus, when better known, may be of some 

 stratigraphic value. For instance H. crassipellis (Deflandre & Cookson) has not been 

 recorded from deposits earlier than the Cenomanian, nor has it been recorded from 

 the Ypresian of England. Thus its stratigraphic range appears limited. Similarly 

 H. cornuta (Gerlach) has not been recorded from deposits earlier than Tertiary. 



Species of Hystrichosphaera are extremely rare in the Upper Jurassic, becoming 

 more common in the Lower Cretaceous and from the Upper Cretaceous to the 

 present day are an important constituent of the dinoflagellate cyst population. 

 Variation has, however, been on a rather limited theme so making species differen- 

 tiation extremely difficult. Moreover there appears to be a complicated plexus of 

 evolution, particularly noticeable in the H. ramosa group. More detailed studies in 

 the future may, however, throw light on some of these problems and aid the syste- 

 matist and the stratigrapher. 



