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



intervening tubes occupy the space of 2 millim. measured 

 transversely. The zooidal tubes are crossed by well-developed 

 transverse partitions or " tabulae," which are often placed at 

 the same level as neighbouring tubes, and thus appear to be 

 continuous. 



Obs. In the general structure of its skeleton S. borealis, as 

 I have elsewhere pointed out (Mon. Brit. Strom, p. 175), is 

 very closely related to S. Gartert, Nich. ; but it seems to be 

 sufficiently separated from this as a distinct species by the 

 following characters : — 



a. The coenosteum is a thin laminar expansion, with a 

 basal epitheca, and not composed of definite latilaminae. 

 On the other hand, S. Carteri is a massive form, with 

 a marked latilaminar mode of growth. 



b. Astrorhizas are numerous and well developed, whereas 

 these structures are wanting altogether or are quite 

 rudimentary in 8. Carteri. 



c. The zooidal tubes have numerous tabular which are 

 often placed at the same level in contiguous tubes, so 

 as to give rise to the appearance of successive con- 

 tinuous concentric lines. 



Formation and Locality. Silurian (Upper Oesel formation), 

 Kattripank, Oesel (coll. H. A. Nicholson). 



Actinostroma expansmn, Hall and Whitf., sp. 

 (PI. X. figs. 1 and 2.) 



Stromatopora e.rpansa, Hall and Whitfield, 23rd Ann. Rep. on the 

 State Cabinet, p. 226, pi. ix. fig. 9 (1873). 



The ccenosteum in this species forms " large expanded 

 masses, sometimes of many feet in extent, with a slightly 

 uneven or undulating surface, which is covered with broad 

 low prominences, distant from centre to centre | to ^ inch " 

 (Hall and Whitfield, loc. cit.). In spite of the presence of 

 the prominences above spoken of, astrorhizas are practically 

 absent, being either quite unrecognizable or being represented 

 merely by one or two pores of larger size than the zooidal 

 apertures. 



The skeleton-fibre is solid and of medium thickness. Tan- 

 gential sections (PI. X. fig. 1) show the typical " hexacti- 

 nellid" structure of the genus Actinostroma — the radial 

 pillars, as seen in cross-section, being rounded or subangular 

 and being united by radially disposed connecting-processes or 



