186 MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 



arising from just within the plate boundary and usually forming a simulate complex 

 on each plate. The processes of each complex are united distally by a membrane 

 which is finely perforate and which assumes the outline and area of the underlying 

 plate. The general rule that the longer the processes, the further away they are 

 from the horns, is found to be true for this species. 



The plates, interpreted from the simulate complexes, show considerable variation 

 in size. Of the apicals, plate i' is by far the largest and has a trapezoid outline : 

 plates 2' and 4' are lateral in position and are narrow, being represented by a single 

 row of processes ; whilst the dorsal plate 3' is intermediate in size between 1' and 

 4'. The anterior intercalary plates likewise vary in size and are dorsal in position. 

 Plates ia and 3a are elongate, narrowing apically, whereas plate 2a, the loss of which 

 forms the archaeopyle, is trapezoid, with processes absent from its equatorial boun- 

 dary. The seven precingulars show extreme variation. Plates 1" and 7" are of 

 comparable size with an almost triangular outline. Plates 2" and 6" are extremely 

 narrow and are represented by a single row of processes running along the lateral 

 margin. On the dorsal surface, the three plates 3", 4", and 5" are wider than the 

 others and do not extend as far towards the apex, being restricted because of the 

 anterior intercalaries. Plate 4" is the widest of the precingular series. 



The circular cingulum comprises five plates, three on the dorsal surface, two on the 

 ventral. Each plate has a single row of processes that are united distally as in 

 W. clathrata. Of the postcingulars, plates 1'" and 5'" on the ventral surface are of 

 comparable size and shape (see Fig. 47), whilst plates 2'" and 4'" are smaller though 

 of not dissimilar outline. The widest of the postcingulars is plate 3'", which usually 

 has no processes on the side adjacent to the transverse cingulum. This is also 

 frequently true of most of the pre- and postcingular plates. The two antapical 

 plates are dorsally situated. The sulcus is considerably expanded on the hypo- 

 tract and extends to the distal extremities of the antapical horns. There can be 

 three or four sulcal plates, of which the most posterior is the largest and has a 

 rhomboidal shape. Only one sulcal plate is present on the epitract. The tabulation 

 is always clearly shown and easily decipherable. 



The width of the pericoel has no bearing on the thickness of the capsule wall, in 

 this or any other species of Wetzeliella. Thin walled capsules often lie at a considerable 

 distance from the periphragm. Evitt's (1961c) hypothesis that the greater the 

 distance of the capsule from the enclosing periphragm, the thicker the endophragm, 

 must therefore be applied with caution. The capsule of W. coleothrypta may be 

 slightly granular ; it possesses an archaeopyle which is in line with that of the 

 periphragm and appears to be intercalary. The operculum of the archeopyle is 

 often found lying within the capsule. 



Remarks. Of the described species of Wetzeliella, only W. clathrata has processes 

 united distally. However, whereas in W. clathrata the processes are united so as to 

 give lists or bars of restricted width, in W. colethrypta, the membrane assumes the 

 outline of the plate and passes completely over it, forming an outer umbrella. The 

 two species can therefore be easily distinguished. 



