166 Dr. G. J. Hinde on a 



attached together and also to the distal margins of the first 

 radials and anal plate below, and laterally to the upper radials 

 and the two lowest arm-joints on each side, so that they form 

 a compact slightly incurving shield over the area. These 

 plates are not so abundantly provided with spines as the anal 

 and radial plates, and they are usually much smoother. 

 There are from one to five spine-bearing tubercles irregu- 

 larly disposed on each plate. The anal aperture is concealed ; 

 it is probably slightly above the third zone of plates. 



Interbrachials. — The space covered by these plates is from 

 8*75 to 10*5 millim. in width in different specimens ; it also 

 varies somewhat even in the same example ; the plates reach 

 from 6 to 95 millim. above the distal margin of the first 

 radials. The plates of the lower of the three zones are always 

 three in number ; they are all larger than the plates of the 

 higher zones, and the middle of the three is very prominent. 

 This is usually, but not invariably, situated centrally above 

 the distal margins of the first radials ; its base is generally 

 straight, but in the rare cases in which there is an entering 

 angle at the union of the first radials the base of this plate is 

 angular and accurately fits into the space. Strictly, there- 

 fore, tills middle plate of the lower zone is an interradial 

 plate. This plate is from 3 to 4 millim. in width and about 

 4 in height. The lateral plates of the lower zone are rela- 

 tively wider and shorter than the central plate ; their outer 

 margins are suturally attached to the sides of the second and 

 axillary radials, even sometimes reaching to the lowest arm- 

 plate. The middle zone consists of three or four polygonal 

 plates, varying from 2 to 3 millim. in width and height, 

 which dovetail into the open angles of the lower series, and 

 their own summit-angles are similarly filled by the top series, 

 usually of four plates. In one specimen there are one or two 

 partially detached plates above the third zone ; but I cannot 

 tell whether such were generally present. 



These interbrachial plates, like those of the anal area, have 

 comparatively smooth surfaces ; in some no tubercles can be 

 detected, probably from obliteration, whilst in others there 

 are from one to three spine-bearing tubercles. The plates are 

 about '5 millim. in thickness, and their sutural surfaces are 

 furnished with longitudinal minute ridges, of which there 

 are from four to five. In none of the specimens is the con- 

 tact of the distal margins of these plates with the plates of the 

 vault actually shown ; but the vault-plates are in such close 

 proximity (PI. VI. fig. 10) that there can be no doubt of 

 their original union with them. 



Vault. — In none of my specimens is the vault complete; 



