imperfectly -known Species of Stromatoporoids. 5 



tial sections (PL I. fig. 6) exhibit the cut ends of the irreguhir 

 radial pillars and the divided edges of the laminse, but show 

 no characteristic feature. 



Obs. As before remarked, this species is closely allied to 

 C vesiculosum, Nich. & Alur., and forms intermediate between 

 the two are not uncommon. The character which most 

 decisively distinguishes C. variolare from the latter is the alter- 

 nation 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. 



Formation and Locality, So far as known^ this species is 

 wholly Silurian {Upper Silurian). It occurs abundantly in 

 the Silurian Limestones of Esthonia, and I have collected 

 many examples from Kattentack and between Saage and 

 Ridaka. The species also occurs in the Wenlock Limestone 

 of Britain (Ironbridge and Dormington) ; but some of the 

 British specimens which I should be disposed to place here 

 differ in certain respects from the Esthonian type. Von 

 Rosen's type-specimen (which I have examined) is from 

 Errinal, in Esthonia. 



Clathrodictyon Linnarssoni, Nich. (PI. I. figs. 7 and 8.) 



Coenosteum laminar, often of considerable size, with a finely 

 wrinkled basal epitheca, the upper surface being smooth and 

 seemingly without " mamelons." Average thickness in the 

 centre of the coenosteum about 1 inch (in the specimens 

 examined). Small astrorhizai are present, with wall-less ver- 

 tical canals ; but these do not open above by projecting 

 apertures. 



In internal structure the coenosteum is composed of close- 

 set horizontal lamince, about six to eight of these occupying 

 the space of 1 niillim. The laminae may undulate gently, 

 but are not minutely crumpled. The interlaminar spaces are 

 crossed by numerous, small, straight radial pillars, which 

 often do not reach the whole way from one lamina to another, 

 and never extend out of their own interlaminar space. The 

 interlaminar spaces are thus broken up into numerous more 

 or less complete cells, which are oblong or square rather than 

 lenticular in form. Tangential sections show the cut ends of 

 the radial pillars and the astrorhizal canals, but ofi"er nothing 

 specially characteristic. Vertical sections (PL I. fig. 7) show 

 further the wall-less axial canals of the astrorhizal, sendino^ 

 off horizontal radiating branches into successive interlaminar 

 spaces. 



