12 Prof. H. A. Nicholson on some ?ieio or 



ingly fine granulations^ without tubercles or " raamelons." 

 Astrorhizse well developed, each system having a vertical, 

 wall-less, axial canal, which opens on the surface of the 

 laminee by a slightly projecting round aperture (PI. III. 

 %.3). 



As regards internal structure, the skeleton is composed of 

 exceedingly delicate concentric laminas, about five of which 

 occupy the space of 1 millim. (PI. III. fig. 1). The laminee 

 are curved, in conformity with the curvatures of the fossil, but 

 are not at all, or but slightly, inflected or crumpled. Each 

 lamina gives off downwards numerous close-set and delicate 

 radial pillars, which may or may not reach the lamina below. 

 The interlaminar cells are thus more or less quadrangular 

 in shape, though often incomplete. Tangential sections 

 (PI. III. fig. 2) exhibit minute rounded or oval dots, often 

 almost linear, representing the cut ends of the radial pillars. 

 When the section coincides with the plane of one of the con- 

 centric laminae these dots are replaced by a delicate reticu- 

 lation. 



Ohs. Owing to dolomitisation, it is very difficult to prepare 

 satisfactory sections of this species^ the minute structure being 

 considerably obscured by mineralisation. The species is, 

 however, clearly referable to Clathrodictyon^ and it is separated 

 from the other species of the genus by the extreme delicacy 

 of the laminae and radial pillars, the form of the interlaminar 

 cells, and the fact that the astrorhizee open superficially by 

 marked and projecting apertures. 



Formation and Locality. The only specimen known is from 

 the Silurian (Guelph formation) of Guelph, Ontario. 



Clathrodictyon laxum, Nich. (PL III. figs. 4 and 5.) 



Coenosteum laminar or incrusting, a basal epitheca being 

 present in the former case. Upper surface smooth or slightly 

 undulated, without " mamelons," and apparently covered with 

 small tubercles. Astrorhizas apparently wanting. 



As regards internal structure, the skeleton is composed of 

 horizontal or slightly flexuous concentric laminae, of which 

 about four are placed in the space of 1 millim. (There are 

 usually three interlaminar spaces in 1 millim.) The laminae 

 (PI. III. fig. 4) are comparatively thin, and are not crumpled 

 or inflected. The interlaminar spaces are crossed by nume- 

 rous delicate vertical radial pillars, most of which reach from 

 one lamina to the next. Tangential sections (PI. III. fig. 5) 

 exhibit the oval or rounded cut ends of the radial pillars along 

 with the siimous edges of the transversely divided laminas. 



