8 Prof. H. A. Nicholson on some new or 



except in Esthonia (in the " Borkholm'sche Schichten "). In 

 the Silurian rocks it is a common species in the Wenlock 

 Limestone of Britain (Dudley, Ironbridge, Dormington, &c.) . 

 I have also specimens from the Wenlock Limestone of 

 Wisbj, Gotland. 



Clathrodictyon crassum^ Nich. (PI. IL figs. 1 and 2.) 



Coenosteum laminar, thin (about a centimetre or thereabouts 

 in thickness), and of small size. The underside is covered 

 with a concentrically-striated epitheca. The upper surface is 

 studded with vermiculate tubercles and exhibits well-marked 

 branching astrorhizal canals. " Mamelons " are not present. 



As regards internal structure, the coenosteum is seen in 

 Vertical sections (PI. IL fig. 1) to be composed of nearly 

 horizontal concentric lamina3, of which five or six occupy the 

 space of 1 millim. The lamina^- are minutely inflected and 

 become blended with the thick radial pillars, by which the 

 interlaminar spaces are divided into irregularly-sized oval or 

 rounded cells. Li tangential sections (PI. II. fig. 2) the cut 

 ends of the thickened radial pillars are seen to form a sort of 

 loose reticulation, in which the ends of the smaller pillars 

 usually appear as dark dots. Such sections also show nume- 

 rous, large-sized, branching astrorhizal canals. The skeleton- 

 fibre is of unusual thickness, and the width of the interlaminar 

 spaces is therefore proportionately reduced as compared with 

 the width of the concentric laminee. 



Ohs. "Without entering here into minute details, it may be 

 stated that C. crassum is most nearly related to C. variolarey 

 Eosen j and forms intermediate between the two are not 

 unknown. Typical examples of the present species can, 

 hoAvever, be at once distinguished by the coarse tuberculation 

 of the surface and the comparatively large development of the 

 astrorhizee, by the excessive thickness of the skeleton-fibre, 

 by the complete reticulation of the laminaj and radial pillars, 

 and by the characteristic aspect of tangential sections. 



Formation and Locality. Rare in the Wenlock Limestone 

 of Britain (Dudley, Ironbridge, and Dormington). 



Clathrodictyon fastigiatum, Nich. 

 (PL II. figs. '6 and 4.) 



Clathrodictyon fostiffiatum, Nicholson, Men. Brit. Strouiatoporoids, 

 p. 43, fig. 3 (figure ouly). 



Ca'nosteum laminar and cake-like, of variable size, but of 



