Noordelijke zwerfsteenkoralen (11)

In the Saalian boulderclay of the northern Hondsrug in the north of the Netherlands, boulderspecimens of the tabulate genera Parastriatopora SOKOLOV, 1949; Thamnopora STEININGER, 1831 and Cladopora HALL, 1851 can be found occasionally; From Parastriatopora SOKOLOV two main types can be distinguished, i.e. (1) more or less regular shaped, cylindrical fragments, with strongly thickened periferal walls and tabulae, forming a rather broad stereozone. In transverse sections the radiate pattern of the corallites is, due to the rather abruptly strong skelettal thickening, often difficult to observe. These boulderspecimens resemble most P. mirifica KLAAM. and P. mutabilis (TCHERNYCHEV). (2) Specimens with a much smaller stereozone, due to the much lesser and more gradually thickened periferal walls and tabulae. Sometimes two or even more successive stereoplasmatic rings can be observed, each separated by non-thickened coralliteparts. In transversie sections a radiate corallitepattern is clearly shown. The axial parts of the branches, containing polygonal corallites are proportionally smaller than the specimens from (1); the shape and diameter of the corallites beying more irregular and varieting. These types resemble strongly Parastriatopora celebrata KLAAM. It has been shown that some irregular or branched and even globular favositids (Hemse-Beds, Gotland) also have developed a stereozone, whereby the central polygonal corallites show the same peculiar 'thamnoporoïd' characteristics as in the Thamnoporina. Most of these favositids cannot actually be separated from those types, which resemble P. celebrata KLAAM. They all seem to be a variable serie of types most probably bound to the same ecology. Also from Thecia MILNE EDWARDS et HAIME, 1849 boulderspecimens are known, which show a well developed stereozone and even 'thamnoporoïd' central corallites.