REPORT OF THE DIRECTOR I915 I29 



we have numbered one series of these plates and in interradius e 

 two others. The oral armature is therefore very primitive. The 

 first floor plates have been modified by a marked increase in size 

 and their acute proximal angles form a well-marked inner jaw. 

 The form of the interradial cover plates is also modified to form 

 a very effective outer jaw. We must note particularly the pair 

 shown in interradius a. While situated over a large axillary infra- 

 marginal (see section shown in interradius d) they maintain close 

 contact with each other and yet show distinct contact with a central 

 transverse ridge on the first floor plates. Their attachment to 

 both the axillary inframarginal and the first floor plates is also 

 shown in plate i, figure i. 



Interesting sections of more distally placed cover plates are 

 shown in plate 9. In figure i, the right cover piece shows a portion 

 of its distal central pit. In figure 3 the section passes close to 

 the narrow, transverse ridge which runs from the position of an 

 inner oral spine to the middle of the outer surface of the plate. 

 It appears here to be an easily detachable surface feature of the 

 plate. Several of these ridges are seen in the upper left of this 

 figure and also in figure 2. It is these ridges which present the 

 coinlike appearance so manifest in plate 8, figure i. The spines 

 borne by these cover plates are of exceptional interest, but we 

 shall better understand their nature if we postpone their con- 

 sideration until after the spines of the remaining arm plates have 

 received our attention. 



The Inframarginals 

 In U. medusa, plate i, figure i, we are viewing two axillary 

 inframarginals at an angle which shows both their proximal and 

 apical faces. The proximal faces are concave and show the 

 impression of plates of the apical skeleton. In plate 2, figure i, 

 the aspect is purely apical. Unlike the arm inframarginals, these 

 plates bear no horizontally placed spinous processes but are 

 markedly concave distally. There can be no doubt therefore that 

 the radial diameter of the plate is here fully shown. That we have 

 also the full lateral diameter will be seen if we note that these 

 plates, in their apical aspect, overlap both the first floor plates and 

 the arm marginals next to them. The radial diameter here is but 

 .64 times the transverse diameter. In plate 10, interradius d, the 

 overlap of the first floor plates has been largely cut away and 

 the horizontal section of the axillary inframarginal is roughly 



