156 BRITISH STROMATOPOROIDS. 



Obs. — Clathrodidyon regulare, Rosen sp., is readily recognised by its slightly 

 inflected, thick laminae, its stout radial pil ars, the oblong, superiorly convex form 

 of the very regularly disposed interlaminar cells (as seen in vertical sections), and 

 the pi-esence of a limited number of radiating " arms " connecting the radial 

 pillars. In its general characters, both external and internal, it most nearly 

 resembles G. striatellum, D'Orb, sp., with which alone it could possibly be 

 confounded. It is, however, an altogether smaller form ; its skeletal tissue is 

 much finer, and the form of the radial pillars and interlaminar cells is quite different. 



Distribution. — This species is of rare occurrence in the "Wenlock Limestone 

 of Britain, and I have not seen any specimens from any locality except Dudley. 

 It occurs also in the Wenlock Limestone of Wisby, Gotland. Von Rosen's 

 orio-inal specimen (which I have examined) is from the Silurian (" zone of Penta- 

 mems esthonus ") of Kleine-Ruhde, Esthonia. 



15. Clathrodictyon striatellum, D'Orb. sp. PI. I, fig. 1 ; PI. V, fig. 3, and 



PL XIX, figs. 6—12. 



Steohatopora concenteica, Lonsdale. Silurian System, p. 680, pi. xv, fig. 31, 

 1839. 



— stkiatella, D' Orbigny. Prodrome de Paleontologie, t. i, p. 51, 



1850. 



— mammillata, Fr. Schmidt. Sil. Form, von Ehstland, p. 232, 1858. 



— — von Rosen. Ueber die Natur der Stromatoporeu, 



p. 71, pi. viii, figs. 1—5, 1867. 



— — Ferd. Boemer. Lethsea Palaeozoica, part 1, p. 531, 



fig. 125, 1S83. 

 Clatheodictton striatellum, Nicholson. Ann. Nat. Hist., ser. 5, vol. xix, p. 6, 

 pi. i, figs. 9 and 10, 1887. 



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

 concentrically wrinkled epitheca. The surface is more or less undulated, but 

 without definite eminences or " mamelons," the concentric laminae usually exfolia- 

 ting concentrically round elevated points (Plate XIX, fig. 6). In well-preserved 

 examples, the surface shows innumerable minute rounded tubercles, between 

 which are small circular or oval pores (Plate XIX, fig. 7). Astrorhizae are appa- 

 rently wanting. 



As regards internal structure, vertical sections (Plate XIX, fig. 8) show that 

 the concentric laminae are comparatively remote, about four interlaminar spaces, 

 and therefore five laminae, occupying the space of 1 mm. ; but the inter- 

 laminar spaces are wider over the convexities of the undulated laminae. The 

 concentric laminae are thrown into successive undulations, which are more pro- 



