Prof. W. King on Pleurodictyum problematicum. 135 



Within the central area, there winds tortuously through the 

 substance of the cell-walls the vermiform appendage already 

 noticed. In thickness it sometimes equals the width of the 

 cell-bases. The best way of examining this structure is on 

 a cast of the under surface of the fossil, similar to that repre- 

 sented in PL X. fig. 1, where it may be seen in the centre fol- 

 lowing a course resembling the letter S through the interspaces 

 separating the casts of the cells. Neither of its terminations is 

 seen on the surface under consideration, as both are continued 

 with a slight outward curve down to the opposite or adherent 

 surface, completing as it were the form of the letter, but having 

 the tails bent on one and the same side of its plane. Another 

 mode of describing this appendage may be adopted by restoring 

 the fossil in imagination, and placing its upper surface before 

 the eye. Commencing at one of its terminations, which is situated 

 about midway between the margin and the centre, it may be 

 traced passing down among the cells with a sinuous curve to the 

 basal plate, across the central part of which it makes a sigmoid 

 flexure ; it next passes with another sinuous curve to the surface, 

 reappearing at about the middle of the side opposite to where it 

 entered. The course of the appendage is often difficult to trace; 

 but I have no doubt that the one just described, allowing for 

 some slight deviations, will be found to be approximately correct. 

 I cannot positively state that I have made out the form of its 

 terminations, though one of them, in a specimen before me, has 

 some appearance of being constricted just immediately before it 

 appears at the surface : with this exception, the thickness of the 

 appendage, throughout its entire length, appears to be remark- 

 ably uniform. 1 also observe that its thickness bears a compa- 

 rative proportion to the size of the fossil, the largest specimens 

 having the thickest appendage. 



A remarkable circumstance respecting the course of this struc- 

 ture remains to be noticed. In half the number of specimens 

 before me, it is folded, where in contact with the basal plate, in 

 the written form of an s ; but in the other half, the letter, as it 

 were, is exactly reversed. In the specimens represented under 

 PI. X. figs. 1 & 4, it may be said to be normally folded ; whereas 

 in Lyell's figure referred to it is inversely folded. 



Connected with the statement that the appendage passes 

 through the substance of the cell-walls, I may add, that it never 

 passes through the cells themselves : those adjacent are short- 

 ened and variously altered in shape to suit its form, as may be 

 seen represented in PI. X. figs. 2 & 9. This is also the case 

 with the foramina; for their thread-like casts, when adjacent 

 to the appendage, never appear as if passing into it, except in 

 what are obviously accidental cases ; they are always to be seen 



