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



Stromatopora hudsonica, Dawson, sp. 

 (PI. VIII. figs. 1-3.) 



Caunopora hudsonica, Dawson, Quart. Journ. Geol. Soc. vol. xxxv. 



p. 52, pi. iv. figs. 9 a and 9 b, and pi. v. fig. 10 (1879). 

 Stromatopora hudsonica, Nicholson, Mon. Brit. Strom, p. 172 (1891). 



Ccenosteum apparently massive, splitting easily into con- 

 centric strata, but not perfectly latilaminar in growth. The 

 base and actual surface are unknown. Surfaces exposed by 

 concentric fractures (PI. VIII. fig. 3) show numerous minute 

 and low protuberances or " mamelons," which are placed 

 about 3 millim. apart, and each of which represents the 

 centre of a small astrorhiza. The astrorhizfe are disposed in 

 vertical systems, each of which is furnished with an axial 

 wall-less tube, the aperture of which is placed at the summit 

 of one of the small " mamelons " above mentioned. The 

 intervals between these are occupied by innumerable small 

 pores representing the mouths of the zooidal tubes. Though 

 the actual surface has not been observed, it may be taken as 

 certain that its characters would be the same as those just 

 described as distinctive of a concentric lamina within the mass 

 of the ccenosteum. 



The skeleton- fibre is minutely porous and about £ millim. 

 in diameter, the skeletal tissue being completely reticulated 

 and constituting a fine and close network. Tangential sec- 

 tions (PI. VIII. fig. 1) exhibit the general skeletal network 

 traversed by the branching horizontal canals of the astro- 

 rhiza 1 , and perforated by minute pores representing the trans- 

 versely divided zooidal tubes. Vertical sections (PI. VIII. 

 fig. 2) show recognizable radial pillars, about eight of which, 

 with their intervening zooidal tubes, occupy the space of 

 2 millim. measured transversely. The zooidal tubes commu- 

 nicate freely by lateral apertures, and are furnished with few 

 transverse partitions or tabulae. Vertical sections also show 

 very conspicuously the vertical wall-less axial canals of the 

 astrorhizal systems. 



Obs. This species, as I have elsewhere pointed out (Mon. 

 Brit. Strom, p. 172), is very closely related to S. typica, 

 Rosen, from which it is distinguished by the following more 

 important characters : — 



a. The astrorhizEe are regularly arranged in vertical 

 systems, each system having an axial wall-less canal 

 of comparatively considerable size. 



b. As the result of the above, the surface shows nimic- 



