48 PROF. P. M. DTJNOAN AND MB. W. P. SLADEN ON 



The transverse sutures of the interradial coronal plates near 

 the apex are well marked with knobs on one side and with pits 

 on the other. The dowelling is as complete as in any Temno- 

 pleurid. But lower down and near the ambitus, the sutural 

 arrangement ceases. We have not been able to see this knob- 

 and-socket structure in the ambulacra, for we have not been able 

 to sacrifice an ambulacral area on purpose. 



The adoral faces of a basal plate and of the contiguous radial plate 

 are covered with knobs ; but it does not appear that this is 

 universal, for whilst in some interradia the adoral faces of the 

 transverse sutures of the plates close to the apex are knobbed, 

 others have sockets. 



VIII. The Badial Flates. 



On carefully denuding one of the radial plates of the specimen 

 preserved in alcohol, the structure of the adoral edge of the plate 

 was rendered visible. The adoral margin of the plate is not a 

 perfect arc, for there is a protuberance in the median line which 

 has a triangular piece underneath it, the outer edges of which 

 are in contact with the aboral margins of the first poriferous 

 plates of the ambulacrum. The protrusion has a broad base, 

 adoral to a glassy tubercle, and there is a narrow and long pro- 

 cess in the median line which, with the triangular piece, forms the 

 centre of the adoral margin of the plate. A very minute optic 

 pore is seen on either side of the central process, close to the 

 free edge of the plate. When the radial j)late is examined from 

 within, there appears a deep pit close to the adoral edge, and it 

 is evident that the nerve divides and that, as in the fossil forms, 

 there are two optic pores which open obliquely, and do not appear 

 on the outer or coloured surface, but on the adoral edge of the 

 plate. The pores look downwards, or adorally and slightly out- 

 wards. 



The specimens show faint ridges and furrows on the radial 

 plates, crossing to them from the Basals, and interfered with to a 

 certain extent by the baso-radial sutures. A low ridge is also in 

 the median line, and the knob alluded to terminates it. 



Gleuus Aebacia, Gray. 

 IX. Structure of the Ambulacral Plates. 

 In Arhacia, Grray, the ambulacral plates, except in the neigh- 



