178 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Dimensions. Holotype : diameter of central body 47 by 52(x. Broad processes, 

 length up to 30fi, breadth up to 2J\l. Length of slender processes up to 22pL. Range 

 of dimensions observed ; diameter of central body 40-57fx. Length of broad 

 processes 20-30^. Antapical process up to 47^. Width of broad processes 12-27^. 

 Length of slender processes 13-20^. Width of slender processes o-5-4(x. Number 

 of specimens measured, 4. 



Description. H. eisenacki is characterized by the broad processes which taper 

 distally to a restricted opening with entire or serrate margin. Frequently branching 

 off from the large processes are small erect tubules with an open serrate distal 

 margin ; commonly there are three or four on each process. The equatorial 

 processes are simple or branched, slender with slightly expanded distal openings. 

 The four apical plates are rarely present ; the archaeopyle has a broad sulcal notch. 

 In the precingular series of plates, plate 6" and its attendant process are considerably 

 smaller than the other five plates and processes, being comparable in size to the 

 anterior sulcal plate and process. In the postcingular series, plate 1'" and its process 

 are the smallest of the series, the other four plates and processes approaching the 

 precingulars in size. The longest process, which tapers before expanding distally, 

 marks the position of the single antapical plate. This process is usually closed and 

 unbranched. There are six sulcal processes, one very large anterior process and five 

 very slender open or bifid or acuminate processes lying between plates 1'" and 5'". 

 The single posterior intercalary process is little different in size and structure to the 

 slender sulcal processes and occupies a position between 1'" and 1"". The apical 

 processes are usually simple, occasionally branched, tapering and open distally. 



Occurrence. London Clay, Whitecliff and Enborne and the Oligocene of Sam- 

 land, East Prussia (Eisenack 1954). 



Remarks. The authors are indebted to Dr. W. R. Evitt of Stanford University 

 for placing at their disposal camera lucida drawings of the holotype of Hystri- 

 chokolpoma cinctum Klumpp (1953) and the originals of H. cinctum Klumpp of 

 Eisenack (1954) ; they are quite different species. The drawings show that H. 

 eisenacki and Eisenack's (1954) specimens of H. cinctum have similar tabulation and 

 differ only in that the former has tubular branches, and not pointed spines, arising 

 from the broad processes. Both are, however, included within the same species here 

 named H. eisenacki. H. eisenacki differs from H. cinctum Klumpp (1953) in the 

 dorsal terminations of the broad processes which are more commonly branched, the 

 smaller number of cingular processes and the possession of a large anterior sulcal 

 process and plate. 



Hystrichokolpoma eisenacki var. turgidum nov. 

 PI 17. fig. 5 

 Derivation of name. Latin, tuvgidus, inflated, swollen, distended. 



Diagnosis. Central body ovoidal slightly granular, bearing processes of two 

 types, broad sub-conical or bulbose, with wide or restricted distal opening and 

 slender, simple or bifurcate processes open or closed distally. 



