150 BRITISH STROMATOPOROIDS. 



'Silurian Limestones of Esthonia, in the "Zone of Pentamerus esthonus" at 

 Kattentack, and in the " Raikiill-beds " at Raikiill. The prevailing species 

 of Clathrodictyon in the Silurian Rocks of Esthonia is, however, not the present 

 form, but C. variolare, Rosen, and I have not seen any examples of this species 

 from Gotland though I entertain no doubt of its occurrence there. 



10. Clathrodictyon variolare, Rosen sp. PI. XVIII, figs. 1 — 5, and PI. XVII, 



fig. 14. 



Stromatopoba variolaeis, von Rosen. Ueber die Natur der Stromatoporen, p. 



61, pi. ii, figs. 2—5, 1867. 

 Clatheodictyon variolabe, Nicholson. Ann. Nat. Hist., ser. 5, vol. xix, p. 4, 



pi. i, figs. 4—6, 1887. 



The coenosteum in this species is laminar, hemispherical or massive, with a 

 •concentrically wrinkled basal epitheca, and often attaining a large size. The 

 surface may be smooth, or may exhibit numerous " mamelons," which are some- 

 times small and pointed, or, more commonly, low and rounded (Plate. XVII, 

 fig. 14). There are numerous astrorhizse, but these are small in point of size, and 

 do not appear to correspond in any special way with the surface-eminences or 

 " mamelons," when these latter structures are present. Generally, the astrorhizae 

 are arranged in vertical groups or systems, each group having a wall-less axial 

 canal, but this arrangement does not appear to be constant. 



The internal structure of G. variolare is very similar to that of C. vesiculosum, 

 the skeleton being composed of close-set concentric lamina?, which are horizontal, 

 or, more usually, gently undulated, and are minutely crumpled, so as to give rise, 

 along with the incomplete radial pillars, to a fine vesicular tissue. From eight 

 to ten laminae occupy the space of 1 mm. ; but the interlaminar spaces 

 are of unequal size, rows of large vesicles alternating with rows of much smaller 

 cells (Plate XVIII, figs. 1 and 3). As a rule, rows of large vesicles are separated 

 by from one to three rows of much more minute vesicles ; but there may be only a 

 single row of the latter or they may even be wanting in places. Tangential sections 

 (Plate XVIII, figs. 2 and 5) show the cut ends of the irregular radial pillars and 

 the divided edges of the crumpled laminae, but show no characteristic features. 

 The radial pillars, especially in the rows of large cells, are very commonly 

 imperfect, thus allowing contiguous cells to communicate freely with one 

 another. 



Obs. — Clathrodictyon variolare, Rosen sp., is very closely allied to G. vesiculosum, 

 Nich. and Mur., on the one hand, and to C. crassum, Nich., on the other; and it 



