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



These concentrically laminated cylinders surrounding the 

 axial astrorhizal canals are disposed in numbers throughout 

 the general undulated laminee of the skeleton. It is the upper 

 ends of these cylinders which form the " mamelons " on the 

 surface ; and one of the axial canals terminates at the summit 

 of each of the latter. On the other hand, the lateral or radi- 

 ating canals of the astrorhizal systems are very incompletely 

 developed. 



Vertical sections (PI. III. fig. 7) show the flexured laminae, 

 of which from three to five occupy the space of 1 raillim. The 

 laminse give off stout radial pillars, which mostly extend from 

 one lamina to the next. In many cases a larger or smaller 

 number of the radial pillars are placed one above the other in 

 successive interlaminar spaces. This is especially the case in 

 the lamina3 which envelop the axial astrorhizal canals, and it 

 gives to the radial pillars, where it occurs, the appearance of 

 being " continuous." The phenomena presented by vertical 

 sections differ, further, according as the section traverses the 

 astrorhizal cylinders through the centre or through the cir- 

 cumference. 



Tangential sections (PL III. fig. 6) show differences in 

 structure according as they traverse the astrorhizal cylinders 

 or the general interstitial laminated tissue. Where such a 

 section cuts a cylinder, we see the central astrorhizal canal 

 with its enveloping concentric laminas. In the tracts between 

 the cylinders we see the cut ends of the radial pillars as 

 oval or circular dots. The character of these shows that the 

 species is referable to Clathrodictyon and not to Actiiwstroma, 

 no radiating " arms " connecting the radial pillars being 

 recognizable. 



Ohs. In the general construction of its coenosteum, the 

 flexured concentric laminae, the astrorhizal cylinders, and the 

 surface-prominences, this species has the closest possible 

 resemblance to Actinostroma verriicosum, Goldf., sp. The 

 fact, however, that the radial pillars are not " continuous " 

 and the absence of radiating " arms " to the pillars prove 

 that the species cannot be referred to the genus Actinostroma 

 at all. It was originally referred by Dr. Murie and myself to 

 the genus Styhdictyon ; but this reference can undoubtedly 

 not be sustained ; and the essential characters of its structure 

 clearly point to its being placed properly in the genus Cla- 

 throdictyon. From all the other known species of this genus 

 it is sufiiciently separated by the peculiar construction of its 

 skeleton, quite apart from all minute structural characters. 



Formation and Locality. Rare in the Hamilton formation 

 (Devonian), Arkona, Ontario. 



