52 



Dr. F. A. Bather — Eocystis. 



specimens, as preserved, it is about 5 mm. The plates appear to 

 have been very thin, but thickened at the umbo and folds. 



Some recent private correspondence suggests the need for an 

 explanation that, in using the term folds, I have never meant to 

 imply that a plate was subjected to any force which threw it into 

 folds. The stereom was, one presumes, deposited in that shape from 

 the outset. But it followed the course and structure of the stroma, 

 and this was usually affected by mechanical stresses. 



It is quite easy to see how under these conditions there arose the 

 hexagonal plating, with folds at right angles to the sutures, common 

 in normal many-plated Cystids (e.g. Heliocriniis, Aristocystis, 

 Echinosfhmrd) ; and the modifications produced by the development 

 of basal and radial plates, as in Caryocrinus and in normal crinoids, 

 also present no difficulty. But it is not at once easy to see what 

 stresses led to the irregularly pentagonal plates of Eocystis, in which 

 pentactiny is superinduced on trisaetiny (see Figs. 2-6). The 

 following explanation is therefore offered. 



If a piece of soft woven tissue be lightly stretched over a frame, 

 and if a few equal weights be attached to various points in it, then 

 the tissue will be dragged into folds leading from each of these 

 weighted points to those adjacent. The direction and pattern of the 

 folding will depend on the positions of the weight-points. 



Diagram showing the lines of mechanical stress 

 supposed to have produced the Eocystis plan 

 of plate. 



The problem before us, then, is to find the weight-points that 

 would produce the fold-pattern of Eocystis. The three major lines 

 of several plates, if plotted on paper, lead to an hexagonal pattern. 

 This at first sight does not seem hopeful. But we note that the 

 umbones of the plates, instead of lying at the centres of the hexagons, 

 lie at the angles, or, in other words, at the nodes where the main 

 lines of stress meet. This suggests that a hexagonal folding of the 



