182 MESOZOIC AND CAINOZOIC DINOFLAGELL ATE CYSTS 



IX. WETZELIELLA FROM THE LONDON CLAY 



By G. L. WILLIAMS & C. DOWNIE 



INTRODUCTION 



The characteristic Lower Tertiary genus Wetzeliella is among the commonest of the 

 London Clay dinoflagellates. The excellent state of preservation has enabled its 

 tabulation to be determined in many instances and its resemblance to the living 

 genus Peridinium is established beyond doubt. Several new forms have been 

 discovered and it is now possible to give a fuller description of some species already 

 named. 



Genus WETZELIELLA Eisenack 1938 : 186 

 Type species. Wetzeliella articulata Eisenack 1938. 



Emended diagnosis. Body with distinct pericoel and endocoel. Periphragm 

 having a distinctive outline, varying from oval to pentagonal and generally prolonged 

 into an apical horn, two lateral horns and one or two antapical horns. Periphragm 

 may or may not bear intratabular processes. Processes (when present) open proxi- 

 mally, open or closed distally and frequently arranged in process complexes. Endo- 

 phragm circular to ovoid in outline, in cross section biconvex and separated, by 

 pericoel of variable size from periphragm. Reflected tabulation of 4', 3a, 7", 5'", 

 2"", 3-4S, ?c not always evident. Cingulum slightly laevo-rotatory, running round 

 maximum width of periphragm. Sulcus wider and longer on hypotract than epitract. 

 Archaeopyle usually present in periphragm and resulting from loss of plate 2a. 

 Endophragm usually with archaeopyle in analogous position. 



Discussion. Wetzeliella is related to the living genus Peridinium, since it possesses 

 identical tabulation and the same type of archaeopyle. It is distinguishable from 

 Peridinium by the presence of usually well developed lateral horns, and in most cases 

 by the numerous processes on the pericoel and the easily recognizable endocoel. 

 Eisenack's (1964) placing of Wetzeliella and Deflandrea in a separate sub-order from 

 Peridinium appears to be an artificial classification which disregards the evidence of 

 tabulation. Dracodinium Gocht, 1955, is no longer recognized as a separate genus, 

 since all stages of transition from Dracodinium solidum, the sole species of the genus, 

 to Wetzeliella similis (Eisenack) occur in the London Clay. D. solidum (pars) is 

 therefore placed in the genus Wetzeliella. Gocht's statement that the position of the 

 slip hole (archaeopyle) is variable, prevents the complete incorporation of Draco- 

 dinium solidum in Wetzeliella. Two sub-genera of Wetzeliella are recognized, 

 Wetzeliella (Wetzeliella) Eisenack and Wetzeliella (Rhombodinium) (Gocht) Alberti, 

 1961. 



The tabulation, which in many species is hard to determine, has been worked out 

 for W. articulata, W. clathrata Eisenack, W. coleothrypta sp. no v., W. reticulata sp. nov. 

 W. tenuivirgula sp. nov., W. homomorpha Deflandre & Cookson 1955, W. condylos sp. 

 nov., and W. similis Eisenack 1954. In the sub-genus Wetzeliella (Rhombodinium) 

 usually the only guide to tabulation is the archaeopyle. 



