Prof. W. King on Pleurodictyum problematicum. 133 



characters give a vertical section of it a somewhat crescentic 

 shape, as may be seen by a reference to Plate X. fig. 2. 



Internally it consists of a number of short, rather wide, sub- 

 polygonal cells, with generally four sides, subradially and alter- 

 nately arranged, and arising from a thin, nearly circular basal 

 plate thickest in the centre and slightly wrinkled concentrically. 

 According to Edwards and Haime this part is covered with epi- 

 theca. By means of the basal plate, a Pleurodictyum attached itself 

 to some foreign body, — at Daun, generally to the convex valve of 

 a Chonetes ; but in process of growth it spread beyond the 

 margins of the shell. Impressions of this part often exhibit 

 subradial and anastomosing lines, which are nothing more than 

 marks of the cell-walls arising from its thinness. An impression 

 of the basal plate with its subradial and anastomosing lines, and 

 a cast of the Chonetes valve to which it was attached, are repre- 

 sented in PL X. fig. 3. 



The cells, of which there appear to be between sixty and 

 seventy in a full-sized specimen, are nearly upright in the central 

 portion, exterior to which they become more and more inclined 

 towards the margin, where they are horizontal or nearly so (vide 

 PI. X. figs. 2 & 8). This modification gives rise to a pecu- 

 liarity of form requiring to be noticed : the central cells * stand 

 on a true base ; but those situated outside of them lose it gra- 

 dually by one of their side walls (the outermost) becoming insen- 

 sibly converted into a base; — the marginal cells, in short, repose 

 completely on one of their side walls (vide PI. X. fig. 2). All 

 the cells, except those in the centre, have their mouth directed 

 towards the margin. 



The cell-bases are usually somewhat rhombic or oval, with 

 the longitudinal axis directed from the centre to the margin of 

 the fossil (vide PI. X. fig. 1) ; and they are generally ^th of an 

 inch in length, and -r^ths in width. 



All, except the marginal cells, have the inner surface of their 

 base marked with a central prominence, which becomes converted 

 into a median ridge in the submarginal cells. The prominences 

 and ridges have produced those large depressions seen on casts 

 of the cell-bases (vide PI. X. figs. 1 & 4). 



The cells are marked interiorly with a number of fine longi- 

 tudinal costules (generally five or six on each of the four walls), 

 which are spinulose on and near the base, and ^j/am on the sides, 

 where their linear arrangement is most decided. On the base 

 the costules are nearly obsolete ; but their spinules are strongly 



* It will simplify my description by designating the cells confined within 

 the area occupied by the vermiform appendage— cew/raZ cells, — those on 

 the margin — marginal cells, — and those between both of the latter — sub- 

 marginal cells. 



