128 NEW YORK STATE MUSEUM 



the apical end of the plate bears a central ridge showing an acute 

 angle on cross section, the oral portion of the inner surface is 

 ventricose and between these ends the plate seems to be rather 

 deeply excavated. The character of the outer surface of a series 

 of six is shown in plate 6, figure i. Note that the apical ends 

 where they meet the floor plates may be either flat or convex. Each 

 cover plate meets two floor plates, one by its apical face and one by 

 a portion of its proximal face. The plates are thus set alternately 

 with the floor plates. They are also imbricated with the oral face 

 tipped toward the mouth. Figure 2 shows four of the same plates 

 from an apical aspect. 



That these plates, though bound together serially, could still act 

 as cover pieces is shown in plate 8, where the food groove is pre- 

 sented in open, partially closed and completely closed positions. 

 The closing of one side of the arm may be also seen in plate 11, 

 figure I. This plate shows a distal, somewhat central concavity 

 in the last cover plate in the lower corner of figure 2, and the same 

 concavity is also revealed in the upper half of the right column in 

 figure I. Sections revealing sigmoid flexures are to be seen in the 

 greatly relaxed position of the upper plates of the left column of 

 figure I, and throughout the left colunm in figure 2. Note par- 

 ticularly the form of the ends where contact is shown with the 

 floor plates as in figure 2. The twist of this portion of the plate 

 apparently allowed each cover piece to rest diagonally across two 

 floor plates and so prevent injury to tissues lying in the transverse 

 cavities of the latter. The extremely relaxed condition shown in 

 this plate is in part due to the beginning of disintegration. This 

 is shown in figure 2 by the displaced floor plate in the upper portion 

 and the appearance of what seem to be two plates of the apical 

 skeleton in the lower part of the figure. It is difficult to believe 

 that the inner faces of the floor plates in this plesiotype had the 

 same depth of those shown in the holotype. It will be easily seen 

 that decay of an open, arm in a form like the holotype, if lying 

 oral surface up, would tend to drag apart the oral portion of the 

 inner faces, of the floor plates and press their apical portions 

 together. 



In plate 10, arm E, disintegration has still further opened the 

 food groove and scattered some of the cover plates. Their alter- 

 nate arrangement is, however, well demonstrated in the upper 

 margin of arm D and we shall be able to demonstrate that for 

 every floor plate there was one cover plate and one alone, even at 

 the very beginning of the development of the arm. In interradius c 



