(80 G. LINDSTRÖM, HELIOLITlDiE. 



cohereiit lamina fringed in tlie edge by short, fiiie spiries ligs. 28 — 30. Their compositioii 

 of rainute, upwards directed fibrillii^ is evident. The tabulw are thin and in-egular, as if 

 bent and broken, and much distantiated. 



The aureola is well developed, the coinpartments short and broad; the radii richly 

 provided vvith lateral thorns and barbs. The width of the aureola corresponds to tlie 

 moiety of the calicle. Outside the aureola there is an abundant coenencliyma far more 

 so than in the preceding species where the calicles are more close. Seen on the surface 

 of the coral it is a network af rounded, irregular meshes Avith funnelshaped bottoms. A 

 little below the surface (fig. 26) almost the same appearance prevails. Deeper down 

 (lig. 27) there is a change and the radii of the aureola are simpler, without lateral barbs, 

 the coenenchyma more regular. In the longitudinal sections the vesicular structure is 

 prevalent, the convex lamellte are of much unequal size and there are only a few aculi=e 

 scattered without order amongst them. Some darker patches may indicate fi-agments of 

 the vertical elements. A few vestiges point to a cancellate arrangement. 



The lig. 25 represents a specimen of coenenchjanal gemmation. The incipient theca 

 enclosing eight or nine rudiments of septa is visible. This interesting species has been 

 found only in the lowest shale beds b and c near Wisby and its neighbourhood, SkälsÖ, 

 and as far north as Halls huk, in south to Gnisvärd, Eskelhem, Djupvik in Eksta, Stora 

 Carlsö and Sproge. Abroad the only known locality, Dudley in England, has contributed 

 with a few specimens, which probably belong to a higher stratum than in Gotland or to 

 the Wenlock limestone. 



Plasmopora calyculata var. tuberosa n. 

 Pl. VII, figs. 2—5. 



The corallum forms irregular tubers of variable shape, covered all around with 

 calicles. These are sometimes a little more than 2 millimeters in diameter. The loall 

 is exsert and composed by the thick exteriör ends of the septa. Instead of the 

 ingoing angle formed at the exteriör septal end at the other species, this angle is here 

 completely filled with calcareous matter in the shape of a transverse rhomboid, giving in 

 combination with the annexed often straight septum the aspect of a little nail. The 

 septa are simple or also provided with latei-al barbs and hooks. They consist of a coherent 

 lamella with edges fringed by small spines and reaching far towards the centre of the 

 calicle. The tabulce are irregular and much scarce. 



The aureola is present, but much irregular and difficult to decipher. In a transverse 

 section it is more evident, its compartments of unequal size, no doubt depending on their 

 having grown on inclined planes. The coenenchyma is abundant and well composed of 

 tubes of variable size and shape. In a longitudinal section these are not so well discer- 

 nible as the large traverses of convex lamella?. 



Of coenenchymal gemmation there are several instances from beginning alterations 

 of the coenenchyma to different stages of the rudimentary theca and septa. 



