158 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



within the genus Baltisphaeridium. A full restudy of this latter genus has recently 

 been published by Staplin, Jansonius & Pocock (1965) ; a consideration of their 

 revisions and proposals is outside the scope of the present paper. 



The Species hirsutum (Ehrenberg) and striolatum (Deflandre). 



Ehrenberg (1838) recorded the occurrence, in Cretaceous flints from Delitzsch, 

 Saxony, of microplankton having simple, oval shells bearing a scatter of simple 

 spines of moderate length. These he named " Xanthidium hirsutum (?) ", thus 

 implying their identity with a modern Desmid, now designated Staurastrum hirsutum 

 (Ehr.) Ralfs 1848. 



Reade (1839) illustrated under this name two quite distinct Upper Cretaceous 

 forms, one having a scatter of long, stout, simple spines, the other a dense matte 

 of very short, fine spines ; neither resembles Ehrenberg's figures at all closely. 

 The second of Reade's forms was also figured, again as X. hirsutum, by White (1842) 

 and has subsequently been designated a distinct species, Baltisphaeridium whitei 

 (Deflandre & Courteville 1939) Downie & Sarjeant 1963. 



Pritchard (1841 : 187, pi. 12, fig. 512) figured as X. hirsutum a fourth morphologi- 

 cally distinct form from the Upper Cretaceous globular, and with a very sparse cover 

 of short, stout spines. This clearly represents a distinct species, but it has not been 

 redescribed and the holotype is lost. 



In 1932, O. Wetzel illustrated a form from the Baltic Upper Cretaceous, which he 

 named Hystrichosphaera hirsuta forma minor (pi. 3, fig. 13) ; this was described and 

 refigured by him (1933 : 91, pi. 4, fig. 26). It was of small size (shell diameter 

 24-28^) with numerous (50-60) simple, stout spines of moderate length, quite 

 comparable to Ehrenberg's figure. Forms from the Dutch Upper Cretaceous were 

 described and figured as forma minor by de Wit (1943 : 381-83) ; his text-figure 

 ga corresponds broadly to Wetzel's description, but his text-figure gb shows a form 

 with very numerous, extremely abbreviate spines, quite unlike Wetzel's description 

 and constituting yet a sixth morphological type! 



Wetzel also described and figured a second form, which he named H. cf. hirsuta 

 forma varians (1932, pi. 3, fig. n ; 1933 : 93, pi. 4, figs. 27-29). This has quite 

 long spines, sometimes branching at their tips, and a shell surface bearing a pattern of 

 low ridges. One of his figures (pi. 4, fig. 29) indicates possession of an archaeopyle. 

 Forms from the Dutch Upper Cretaceous were figured under this name by de Wit 

 (1943, text-fig. zoa, b). 



In 1937, Deflandre transferred the species hirsutum to his genus Hystrichosphaeridi- 

 um, commenting : " It is quite certain that neither of the forms described by 0. 

 Wetzel . . . corresponds to the species of Ehrenberg.". He suggested, but did not 

 firmly propose, elevation of forma minor to specific status, as Hystrichosphaeridium 

 minor. In the same paper, a new Upper Cretaceous species was described, having a 

 surface divided into more or less triangular, concave fields and bearing appendages 

 of very variable character (relatively slender, simple or branching spines, together 



