200 Br. F. A. Bather — Studies in Edrioasteroidea. 



The thickness of the cover-plates about half-way down the r. ant. 

 radius of specimen E is not more than '35 mm. In many cases the 

 cover-plates have been pulled or pressed down into the groove, but 

 when preserved in what would seem to be their normal position they 

 form a prominent rounded arch over the groove. The sutures 

 between adjacent cover-plates, as well as the median suture, are slightly 

 depressed and bordered by spine-pits. The suture between the 

 cover-plates and the adambulaerals is flusli, and the curve of the 

 cover-plates passes over, though with a distinct bend, into that of 

 the adambulaerals. The suture is not a straight line, but a series of 

 curves, the convex outer edges of the cover-plates fitting into slight 

 concavities in the adambulacral margin. The position and number of 

 the axial ridges on this margin indicate that the original adambulacral 

 elements coincided in number but alternated in position with the 

 cover-plates, and therefore also with the floor-plates. This suture, 

 then, is essentially a zigzag suture between two sets of alternating 



arms bydropore. 



Fig. 5. Steganoblast'us ottaioaensis . Specimen B viewed from 

 above, for comparison with PI. XV, Fig. 5. X 4 diam. 



plates. In consequence of this arrangement, one would expect to 

 see along the edges of the groove, when the cover-plates are 

 removed, a series of depressions or facets for the reception of the 

 cover-plates. Unfortunately the edges have in nearly every case 

 been worn enough to remove all trace of these very faint depressions, 

 and it has only been by dissecting away some cover-plates at the end 

 of ther.ant.ray in specimen B that I have been able to distinguish 

 anything of the kind. 



The Tegmen is best studied in specimen A (PI. XY, Fig. 7 ; Text- 

 fig. 6). Here the cover-plates are more regular than in B, and the 

 arrangement of the whole follows an obvious symmetrv, which, from 

 its resemblance to that in other Pelmatozoa, must be regarded as 

 the normal. In addition to the cover-plates, the tegmen contains 



