194 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



Occurrence. Eocene, London Clay ; Sheppey. 



Remarks. Only two described species of Wetzeliella, W. lineidentata Deflandre & 

 Cookson, 1955 (Lower Tertiary, Australia) and W. irtyschensis Alberti, 1961 (Oligo- 

 cene, U.S.S.R.) have processes of a similar nature to those of W. condylos. W. 

 lineidentata was originally based on a single damaged specimen, the apical region of 

 which was absent. Cookson & Eisenack (1961) however, discovered beautifully 

 preserved complete specimens of this species from the Lower Tertiary of Western 

 Australia. W. lineidentata differs from W. condylos in having lateral horns which 

 arise in a medial position, a hypotract and epitract of similar size, the epitract of the 

 outer shell having a triangular outline ; the capsule outline. The two species appear 

 to be closely related however. W. irtyschensis differs from W. condylos in having 

 poorly developed lateral horns and two antapical horns of equal length. 



Wetzeliella {Wetzeliella) similis Eisenack 

 PL 20, fig. 5 



1954. Wetzeliella similis Eisenack : 58, pi. 8, figs. 8-10. 



1 961. Wetzeliella cf. similis Eisenack ; Gerlach : 154, pi. 25, fig. 5. 



Discussion. W. similis is interpreted from Eisenack's diagnosis, as a species of 

 Wetzeliella having a broad low apical horn ; long, drawn out lateral horns ; and one 

 antapical horn, which lies to the right of the midventral line, whilst to the left the 

 other antapical horn is represented only by a low protuberance. The figures 

 accompanying W. similis in Eisenack (1954) are unfortunately too poor to give any 

 further help in recognition of the species, although one specimen he figured (pi. 8, 

 fig. 9) has an apical opening and must be considered to belong to a genus other than 

 Wetzeliella. 



W. similis is a species intermediate between W. articulata and W. solida (Gocht) 

 (pars) Eisenack 1961, the former having a well developed apical and two antapical 

 horns, the latter possessing only one well developed antapical horn, whilst an apical 

 horn is absent. Attempts to set up a varietal name of W . similis for forms with very 

 reduced apical horns have proved fruitless in the London Clay, intergradation being 

 so gradual that it is impossible to distinguish any dividing line. It is therefore 

 considered more advisable to extend the limits of W. similis to include forms with 

 apical horns less than j\t. long. Forms with apical horns below 6[x would be placed 

 in Wetzeliella solida. The apical horn of W. similis usually merges imperceptibly 

 into the outline of the epitract ; it can occasionally be more sharply delimited. 

 The single specimen described and figured by Gerlach (1961) is here included in 

 Wetzeliella (W.) similis. 



Examples of Wetzeliella (W.) similis from the London Clay have an apical horn 

 which is considerably broader and lower than that of W. articulata, whilst the 

 indented lateral horns are long and drawn out. The right antapical horn is always 

 longer than the apical horn. The left antapical horn is represented by a slight 

 bulging of the pericoel. The breadth/length ratio of the outer shell generally 



