GENUS STYLODICTYON. 79 



sections (Figs. 11 and 12, and Plate V, figs. 1, 3, 5, G) the general aspect of the 

 structure resembles that of the vesicular tissue of a Gystiphyllum or the so-called 

 11 coenenchymal " tissue of such corals as Plasmopora or Fistulipora. Tangential 

 sections (Plate V, figs. 2, 4, 7) show a somewhat similar reticular structure, 

 sometimes vermiculate, and generally more or less clearly exhibiting the cut ends 

 of a number of the radial pillars. 



Astrorhizae are generally present. The surface is either minutely granular, or 

 is covered with vermiculate ridges (Plate II, figs. 12 and 13) ; but there are no 

 conical elevations or " mamelons." 



With the exception of G. cellulosum, Nich. and Mur., from the Corniferous Lime- 

 stone of Canada (Fig. 11), and of an undescribed form from the Devonian Limestones 

 of South Devon (from the collection of Mr. Champernowne), I am not acquainted 

 with any Devonian species of Clathrodictyon. On the other hand, the genus is largely 

 represented in the Upper Silurian rocks. The type-species is the C. vesiculosum, 

 Nich. and Mur., of the Clinton and Niagara formation of North America (Plate V, 

 fig. 5). A very nearly allied type is the C. variolar e, Eosen, sp. (Plate V, fig. 6), 

 of the Wenlock Limestone of Esthonia and of Britain. Another very beautiful 

 species is C. striatellum, D'Orb. sp. (Plate V, figs. 3 and 4), which is characteristic 

 of the "Wenlock Limestone of Britain, but occurs also in the Ordovician Rocks of 

 Esthonia, whence it was described by Friedrich Schmidt under the name of Stroma- 

 topora mammillata. Other Wenlock species of the genus are G. regulare, Rosen, sp. 

 (Plate V, figs 1 and 2), and G. fastigiatum, n. sp. (Fig. 12). 



Genus Stylodictyon, Nich. and Mur. 



(< Journ. Linn. Soc.,' vol. xiv, p. 221, 1878). 



Coenosteum massive, traversed by numerous closely-set circular vertical columns 

 of large size, which are formed by the upward bending of the concentric laminae, 

 and which terminate on the surface in small pointed eminences (Plate VII, 

 figs. 7, 8, 9). Each of these vertical columns is composed of a dense central axis 

 surrounded by a zone of thickened reticulated tissue. The intercolumnar spaces 

 (Plate VII, figs. 10 and 11) are occupied by the general tissue of the skeleton, 

 composed of concentric laminae and radial pillars, and much resembling the skeletal 

 tissue in the genus Clathrodictyon. The radial pillars are imperfectly developed, 

 not being " continuous," and commonly falling short of the lamina next above that 

 from which they take their origin. The concentric laminae are well developed, 

 being curved in each intercolumnar space, with their convexities downwards, and 

 bent upwards sharply as they join the " columns '' on both sides. The laminae 

 have further a kind of alternate arrangement in groups, those of one group being 



