imperfectly -known Species of Stromatoporoids. 323 



latilaminar in growth, while the concentric skeletal layers are 

 simply curved and not undulated. The surface is therefore 

 devoid of prominences or " mamelons." As a rule there are 

 well- developed astrorhizse which possess numerous ramified 

 branches and have their centres from 1 to 1^ centim. apart. 

 The astrorhizse do not appear to be superimposed in vertical 

 rows and do not possess conspicuous axial canals. 



The skeleton-fibre is coarse, but is not porous. Vertical 

 sections (PL X. fig. 4) show exceptionally strong radial 

 pillars, very regularly parallel, and running without a break 

 for long distances. About five or six pillars occupy a space 

 of 2 millim. measured transversely. The connecting-processes 

 of the radial pillars are thick and irregular, about seven or 

 eight " concentric laminse " occupying the space of 2 millim. 

 measured vertically. 



Tangential sections (PI. X. fig. 3) show the large circular 

 or oval ends of the transversely divided pillars, joined by 

 thick and irregular connecting-processes and separated by 

 rounded zooidal pores or by branches of the astrorhizal canals. 

 Obs. This species was originally described by me from 

 examples obtained from the pebbles of Devonian limestone in 

 the Triassic conglomerates of South Devon, none of such 

 specimens being more than fragmentary. The collections of 

 the Geological Survey of Canada contain, however, some 

 specimens from the Devonian limestones of Lake Manitoba 

 which appear to be unquestionably referable to A.fenestratum, 

 and which show that the ccenosteum was massive, not lati- 

 laminar, and without astrorhizal prominences. 



A. fenestration is readily distinguished from all the other 

 species of Actinostroma by the very coarse character of the 

 skeletal fibre. In this respect the Canadian specimens — 

 perhaps owing to their mode of preservation in dolomitic 

 limestones — somewhat surpass the English examples, the 

 latter having five or six pillars in 2 millim., while four or 

 five pillars occupy the same space in the former. [It may be 

 noted that tangential sections are not unlike similar sections 

 of badly preserved examples of Stromatopora Beuthii, Barg., 

 but are at once distinguished by the fact that the skeleton- 

 fibre is solid and not porous.] 



Among the American species of Actinostroma the present 

 species stands nearest to A. expansum, Hall and Whitf., from 

 which it is distinguished by its coarser skeleton-fibre, the 

 simply curved laminse, the absence of the flat and rounded 

 "mamelons" of the latter, and the well- developed condition 

 of the astrorhizse. A.fenestratum also presents a resemblance 

 to Syringostroma ristigouchense } Spencer ; but these two forms 



