REPORT OF THE DIRECTOR I915 .I33 



the ray measure at least .75 mm or one and one-half times greater 

 than in U. medusa. . 



We turn now to spinous processes which assuredly belong to the 

 cover plates. In plate i, figure i, arm A, we see a single spine pro- 

 jecting from the inner, oral, proximal corner of a preserved third 

 cover plate. This spine is .08 mm in diameter and shows .3 mm of 

 its length. It is probable that it was one of a pair attached to the 

 inner edge of the coinlike medial ridge of the plate. In plate 2, 

 figure I, the area from which the floor plates were lost shows a 

 diverging pair of spines which apparently belonged to the inner, 

 oral end of cover plate 10. The right-hand spine is seen to fork 

 and show two very delicate spinelets which were bent inward by the 

 soft mud of the bottom and now show cross sections appearing like 

 minute white specks terminating the branches of the fork. These 

 pedicels, for such it seems we must call them, had diameters of but 

 .06 mm and a length (shown by the neighboring pedicel) of at 

 least .22 mm. In U. pulchella, plate 9, figure 3, we find an 

 inner spine about .09 and having a length of .4 mm. This spine is 

 on a fifteenth covering ossicle and yet is very markedly larger than 

 the pedicels in a similar position on a tenth cover plate in U. 

 medusa. This appears to be a simple spine but may also be one 

 of a pair. 



In plate 7, figure 2, upper left, we see several of the medial oral 

 coinlike ridges of U. pulchella. These are in the vicinity of 

 the thirtieth cover plate and where the arm begins to turn over and 

 show its apical surface. The transverse diameter of the ridge on 

 the uppermost cover plate, near its inner end, is .15 mm. This end 

 of the ridge is angulated and bears two articulated spines .08 mm 

 in diameter. The following cover piece shows also the same outer 

 angle with one of a pair of spines still attached. From the evidence 

 we should be inclined to credit each cover piece with an inner, oral 

 pair of articulated spine bases, bearing two spinelets each. 



Turning now to indications of spines on the outer, oral edges of 

 the cover plates, we find them first in plate i, figure 2, where the 

 members of a consecutive series (in upper left of figure) show a 

 single spinous process each. This process is close to one end of the 

 medial, coinlike ridge which we have seen in U. pulchella 

 and may be but the end of that ridge. If these processes are pedi- 

 cels, their position is also such that the bottom muds would turn 

 back their spinelets. Both the twenty-third and twenty-fifth cover 

 plates show a pair of cross sections of such spinelets. Additional 

 white dots along the margin of this arm indicate the presence of 



