152 BRITISH STROM ATOPOROIDS. 



No " mamelons " are present, though the astrorhizse are often superposed in 

 vertical systems. 



As regards internal structure, the coenosteum is seen in vertical sections 

 (Plate XVIII, fig. 6) to be composed of nearly horizontal or gently bent concentric 

 lamina3, of which five or six occupy the space of 1 mm. 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. 

 In tangential sections (Plate XVIII, fig. 6) the divided edges of the interlaminar 

 cells and the cut ends of the radial pillars form a coarse and loose reticulation in 

 which the ends of the pillars are seen as dark dots or granules. Such sections 

 also show numerous 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 laminae. 



Obs. — Clathrodictyon crassum is most nearly related to G. variolare, Rosen, and, 

 as before mentioned, some examples of the latter make a decided approach to the 

 present species. Typical examples of these two species cannot, however, possibly 

 be confounded with one another. As regards its general characters G. crassum, 

 Nicb.., is sufficiently distinguished by its constantly small size and the fact that 

 it grows in thin laminae. The coarse tuberculation of the surface and the 

 comparatively large development of the astrorhizse are also good distinctive 

 characters. The characteristic features of the internal structure are the excessive 

 thickness of the skeleton-fibre, the complete reticulation of the laminaa and radial 

 pillars, and the peculiar coarse granular reticulation presented by tangential 

 sections. The largest specimen I have seen had a long diameter of about 8 cm., 

 and a thickness in the centre of rather more than 1 cm. 



Description. — G. crassum is only known as occurring in the Wenlock Limestone 

 of Britain, in which it is a rare form. I have collected examples at Ironbridge 

 and Dormington, and Mr. William Madeley, to whom I am indebted for much 

 friendly assistance, has been good enough to give me a specimen from Dudley. 



12. Clathrodictyon fastigiatuh, Nich. PI. XIX, figs. 1 — 5. 



Clatheodictyon fastigiatum, Nicholson. Introduction, p. 43, fig. 3 (figure 



only), 1886. 

 — — Nicholson. Ann. Nat. Hist., ser. 5, vol. xix, p. 8, 



pi. ii, figs. 3 and 4, 1887. 



The coenosteum in this species is laminar and cake-like, of variable size, but of 

 small thickness, full-grown examples having a diameter of 15 cm. or more, with a 



