REPORT OF. THE DIRECTOR I915 I3I 



turned in opposite directions but still remaining in close contact 

 with the head of the pedicel. The space occupied by their bases 

 indicates room for a buried third spinelet. In arm C the fourth and 

 fifth marginals each show the basal diameters of single spinelets 

 still practically in contact with their respective pedicels. These 

 diameters are such as to allow the pedicel heads to possess three 

 articular surfaces for such spines, and three only. The gum 

 mounting shows that the spinelets and pedicels are built of an alter- 

 nating series of light and dark discs. The dark discs indicate the 

 former presence of organic tissues, the white discs the presence of 

 more open or spongy stereom formation. The writer would inter- 

 pret this appearance as indicating that the spinelets were increased 

 in length by a series of tissue extensions at the tips, these exten- 

 sions becoming consecutive centers of stereom formation which, 

 however, did not completely join one another. The spines seem 

 also to have been formed in the same manner and the whole struc- 

 ture is of a very primitive nature. 



Such a structure must not only have kept the spinelets from 

 becoming very rigid bodies, but it must also have allowed them to 

 fall apart, like a broken string of beads, in decay. Plate 2 presents 

 evidence to corroborate both suppositions. The upper half of 

 figure 3 shows three spinelets thus falling apart; the middle one is 

 long and contorted. The length of this spinelet must have been at 

 least 15 times its basal diameter. Near the lower part of the figure 

 we find the partially filled mold of a spine of approximately the 

 same length. 



This figure shows that the pedicels also became separated from 

 the plate bases during decay and, like the spinelets, broke up into 

 similar but more robust beads. In the upper right corner of figure 

 3 are seen two pedicel beads, the upper showing a concave face and 

 the lower an apparently convex face. Not far below this pair is a 

 larger portion of a separated pedicel. Its left face is convex and 

 either represents the face to which the spinelets were attached or 

 the outer face of the joint just under the head of the pedicel. 



In plate 5 we will notice first, near the upper left corner, a bent 

 portion of a spinelet breaking up into discs. In the arm margin, 

 above the thirteenth floor plate, the pedicel of an ambital plate is 

 separating into discs. The ambital, seen in side view, immediately 

 at the left of this is crossed by white bands. A separation at these 

 bands would break the pedicel up into four parts. The head of 

 this pedicel is covered with black tufts in the calcite which repre- 

 sent the muscles moving the spinelets ; near the lower left of this 



