84 G. LINDSTRÖM, HELIOLITIDiE. 



thei'e are vestiges of an aureola and 10 — 11 septa. In section vi the right hand calicle 

 again is more advanced, with twelve septa and eleven rudirnentary aureola corapartments, 

 also in section vii. 



There exists inoreover a sort of calicular gemmation, the calicles having grown upwards 

 detached from the ambient coenenchyma, secreting a new coenenchyma around their thecal 

 edge, from which buds grow in accordance with the procedure already described above 

 in Heliolites. 



To be remarked is the pitted surface of some specimens, forraing small polygones, 

 evidently inarking out the boundaries of each single calicle with its interiör area, commonly 

 called calicle, in the centre and the coenenchyma] börder around it as in Pl. calyculata. 

 There are transitions from quite smooth or plane surfaces to such with very shallow pits 

 and at last those with deep ones. 



In a few calicles abnormities have been found with only 8 or 10 septa, the onlj' 

 deviation known amongst the Heliolitida? from the regular number of twelve. 



This species has been found in several specimens on the shores near Wisby, evidentl}^ 

 derived from the lowest Upper Silurian beds of Gotland, the Arachnophyllum stratum, a, 

 and 1 have also a specimen from a nearly corresponding stratum G^ at Kattentack in 

 Estland. In Norway it occurs in the islet Malmökalven in the Christiania-fiord. 



Plasmopora scala n. 

 PI. v, figs. 27—28. 



Corallum disciform, thickness at most 10 millims., with calicles only on one side. 

 Septa reaching nearly to the centrum, equal, simple and straight, forming a coherent 

 lamella with irregular spines in the edges. The tabulce are concave, distantiated. The 

 aureola is regular, in width nearly equal to the diameter of the calicle. Its radii are 

 a little sinuous and provided with short lateral barbs. 



The coenenchyma seen in a transverse section presents the many, small, irregularly 

 polygone tubuli, the walls of which continue downwards, sinuous, interrupted, enclosing 

 horizontal traverses. There are only few convex lamella; besides. 



The chief differences from Pl. stella lie in the structure of the aureolar radii, these 

 being barbed, not smooth, and also in the irregularity of the coenenchyraal tubuli. 



It has been found only in the Ai-achnophjdlum bed a near Wisby. 



Plasmopora rosa n. 



Pl. VII, figs. 21—23. 



Corallum disciform. Calicle scarcely attaining one millira. in diameter, provided 

 with twelve short septa. reaching halfways to the centre, forming a narrow lamella friiiged 

 in the edge and composed in the usual inanner by microscopic iibrillaa directed upwards. 

 The fahula' are horizontal or ol)lique and a littlo concave, closel)' set. The aureola 



