134 Prof. W. King on Pleurodictyum problematicum. ' 



developed, particularly on each side of the prominence or me- 

 dian ridge. The plain costules are most prominent near the 

 margins of the cells, giving the latter a sulcated character. 

 Although, from their being nearly obsolete on the base of the 

 cells, it is difficult to trace the costules on this part, yet there 

 are sufficient indications to show that they radiate from the 

 central prominence or median ridge and pass up the walls. 

 The intervening furrows are plain, except at the inferior half of 

 the cells and the marginal portion of the base, where they are 

 punctured : on casts they appear like granulated lines. The 

 spinules have produced those pricked holes exhibited on casts of 

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



The walls of the cells vary from y'^-th to ^th of an inch in 

 thickness, the thickest portion being at the base. Besides the 

 character last described, they are perforated completely through 

 or foraminated, usually at intervals of about j-^th of an inch. In 

 general the perforations or foramina, which are arranged some- 

 what longitudinally (vide PI. X. figs. 5 & 6), pass direct through 

 the substance of the cell-walls, except at or near their base, 

 where they are long, curved, and sometimes anastomosed [\'\de 

 PI. X. figs. 4 &c.). In their foraminated charactir the cell- 

 walls offer a striking resemblance to those of Favosites and 

 Michelinia. 



Interpolated or young cells are occasionally seen springing 

 from all points of the outermost walls of old submarginal cells : 

 when not in contact with the basal plate they are of a conical 

 form, and their pointed base terminates usually in two foramina 

 which diverge and pass into the adjoining cell or cells (vide 

 PI. X. figs. 2 & 7). 



All the cells, except those in the centre, grew out horizontally 

 for awhile ; but as soon as new cells external to them were de- 

 veloped, their aperture became turned up, giving a genieulated 

 form to their outward outline, as represented in PI. X. figs. 2 & 8. 

 This peculiarity of growth occasionally gave rise to a consi- 

 derable interspace between the mouths of old adjacent cells. It 

 is in this interspace that their interpolated or young suhmarginal 

 cells, noticed in the last paragraph, originated. The develop- 

 ment of the latter did not cause the fossil to increase in height, 

 as might be supposed ; for in no instance have I observed them 

 to rise above the old cells. The increase of Pleurodictyum was 

 by horizontal growth, mainly due to young cells originating on 

 its margin : in their earliest stage, even when scarcely ^th of an 

 inch in length, these young marginal cells are foraminately con- 

 nected with older ones, and their pointed or inferior end is also 

 often seen terminating in two diverging foramina, as obtains in 

 their submarginal analogues. 



