106 NEW YORK STATE MUSEUM 



_ sion of the plate was not so rapid as its upward. It required 

 also the gradual rilling in of the older position of the hydrospire 

 slit or pore. The filled in material did not reach the outer sur- 

 face of the plate and the external furrows thus lie directly over 

 the internal grooves. That the filled in portion was the weaker 

 and more readily dissolved in weathering seems to be shown by 

 the widening of the slits of the lower margin in all plates found 

 in a detached condition. This widening is shown in the figures 

 of plate 5 save in the fragment protected by its bibrachial at j. 



During the downward extension of the older hydrospires there 

 appears to have been a widening of their bases to correspond 

 with an increase of function. This was accompanied not only 

 by the inward bending of the plate toward the center of the 

 theca, of which it may have been in part the cause, but also by 

 the throwing of the outer ridges between the slits into a marked 

 zigzag and giving them a still stronger external thickening near 

 the suture. 



There was no marked thickening of the plate after the addi- 

 tion of new sheets of stereom at the edges. Young plates are 

 somewhat thinner than the old plates but the latter are thinner 

 near their centers. The structure of the ambulacrum, having at 

 least in places, four plates in a line across the edge of the deltoid, 

 necessitated a rather thick sutural face for even young plates. 

 This thickness was gradually increased at the lateral edges of the 

 plate as is shown by figure 2. 



The increased growth at both ends of the hydrospires and the 

 thickening of the plate was accompanied by increased growth 

 in depth of the hydrospire membrane; the oldest portions or 

 that under its place of origin becoming the deepest, hanging 

 far into the coelomic cavity, and giving a triangular outline to 

 the structure when viewed from the side. To enable them to 

 make this continued growth these structures must have had their 

 inner edges remain membranous. No primary calcification is 

 apparent in the cross-section, their outlines being seen only as 

 a fine, rather interrupted line of carbon particles. The portions 

 of the membrane next the plate became strengthened by the 

 deposit of calcareous matter, but the whole structure was so 

 delicate as to be rarely preserved. Plate 5, figure n, has a small 

 area showing the outer edges of these thin hydrospires still 

 attached to the plate. Another fragment of a deltoid shows 

 them in a still more perfect state of preservation but so filled 

 in with rock deposit as to show but little in a photographic 



