In peridinioid dinoflagellates of the Maastrichtian (Upper Cretaceous) of Trelleborg (Scania, southern Sweden) analyses of the length/width ratio of the pericysts and the endocysts of different species also indicate that the variation of the pericysts generally is greater than that of the endocysts. The endocysts display a less elongated shape which is fairly constant within a species. The specific mean values of the length/width ratio of endocysts range between 0.8 and 1.4 with the median value = 1.0, while the specific mean values for the pericysts range from 1.3 to 4.5, with the median value = 2.1
In these peridinioid dinoflagellates the restricted variation in size and shape of the endocysts indicates that the endophragm was developed under strong genetic control inside the motile, vegetative theca. The pericysts, however, display a wide morphologic variation which may be the result of influences by environmentally controlled factors during the development of the periphragm. This development may have taken place also on the outer side of the motile theca, or even after its decay.
Analyses of the Trelleborg material of the length/width ratio of pericysts and endocysts by reduced major axis in scatter diagrams indicate that within a genus the slopes of the regression lines are close to each other within a genus with statistically significant correlation coefficients. Thus the slope of the regression lines may be used for taxonomical purposes.
See also >> Dinoflagellater: mätningar och klassifikation (in Swedish only)