CLATHRODICTYON CRASSUM. 151 



is connected with both of these species by transitional forms. The character 

 which most decisively separates it from G. vesiculosum is the alternation of rows 

 of large cells with wider or narrower zones of exceedingly minute vesicles. In 

 other respects the two forms stand very close to one another. Some of the 

 British specimens are absolutely typical, and differ in no respect from the ordinary 

 Russian examples of the species. Other British specimens which I regard as 

 referable to this species show a marked diminution in the rows of small vesicles 

 which normally separate the rows of large cells. In such cases (Plate XVIII, 

 fig. 4) the vertical section of the coenosteum shows only rows of comparatively 

 large- sized cells with few or no rows of small vesicles. Such examples show an 

 approximation to the type of C. crassum, Nich., this being further accentuated by 

 the fact that the skeleton-fibre is decidedly coarser in these cases than it is in 

 thoroughly typical specimens of G. variolare. The only other species of the 

 genus with which the present form could well be confounded is G. Linnarssoni, 

 Nich. In this latter species, however, the concentric laminaa are not crumpled, 

 and as a result of this the interlaminar vesicles are quadrangular rather than 

 lenticular in form, while the interlaminar spaces are approximately uniform 

 in width. 



Distribution. — G. variolare has not hitherto been detected out of the Silurian 

 Rocks in Britain ; but it occurs in the Ordovician (Borckholm'sche Schichten) at 

 Borckholm and Worms in Esthonia. With this exception its vertical range seems 

 to be much the same as that of the preceding species. I have not, however, met 

 with any examples of it in the Clinton and Niagara formations of North America, 

 where C. vesiculosum is very abundant. Von Rosen's type-specimen of G. variolare 

 (which I have examined) is from Errinal in Esthonia. I have also collected the 

 species abundantly in the " zone of Pentamerus esthonas " at Kattentack in 

 Esthonia, and elsewhere in the same region. In Britain the species is by no means 

 uncommon in the Wenlock Limestone, and I have collected examples of it at 

 Ironbridge, Dormington, and Much Wenlock. 



11. Clathrodictyon crassum, Nich. PI. XVIII, figs. G and 7. 



Clathrodictyon ckassum, Nicholson. Ann. Nat. Hist., ser. 5, vol. six, p. 8, j)l 



ii, figs. 1 and 2, 1887. 



The coenosteum in this species is in the form of a thin laminar expansion (about 

 a centimetre or thereabouts in thickness) and of small size. The under side is 

 covered with a concentrically Avrinkled epitheca. The upper surface is studded 

 with vermiculate tubercles, and exhibits well-marked branching astrorhizal canals. 



