82 



THE BLASTOIDEA 



Codaster, M'Coy (1849), Silurian to Carboniferous, Britain and N. 

 America, is the least specialised of all Eublastoids (Fig. V.). The 3 BB 

 form a conical cup, on which follows the more cylindrical circlet of 5 

 RR, the processes of which bend in 

 above almost at right angles, to form 

 part of the truncated summit or oral 

 surface. Following on the upper mar- 

 gins of the radial processes come the 5 A, 

 which surround the pentagonal mouth- 

 opening (Fig. V. 1). These A probably 

 represent the combined A and pore- 

 plates of Asteroblastus ; like those pore- 

 plates they have a pronounced median Deltoids 

 crest (" oral ridge," Etheridge & Car- Radials f - / |f w 

 penter). Between adjacent A, and >^ 



passing down into the sinus of each Basals 

 R, is a long plate, known from its 

 shape as the " lancet-plate " (Fig. V. 2) ; 

 the edges of this partly overlap the R 

 and A ; its upper surface bears a groove 

 which passes between the adjacent A 

 to the mouth. It is pierced by a cen- 

 tral canal, comparable to the central 

 hollow seen in Mesocystis, and probably 

 filled by a nerve from the aboral 

 system (Fig. V. 6). Along the sides 

 of the lancet-plate lie small "side- 

 plates," not, however, in single series, 

 but in zigzag, so that each pair forms 

 a rhombohedron bisected by the suture 

 between them (Fig. VI.). On these 

 sutures are brachiole - facets ; and 

 branches from the ventral groove pass 

 alternately to the side-plates, and on 

 to the brachioles. The whole groove 

 was covered by small movable plates, 

 the impressions of which are seen along 

 its sides. The peristome was similarly 

 plated over. The apposed edges of 

 the radial processes and *the A are 

 thrown into a set of strongly-marked 

 folds at right angles to the radio- 

 deltoid sutures (Fig. V. 6). These 



folds in the stereom may be an ex- 



; Reconstruction of a typical Eublastoid, 



aggeration OI the axial folds SO con- Orophocrinns (vel Dimorphicrinus) fusifor- 



spicuous in Asteroblastiis ; the stereom 



forms a thin -folded wall, the ends Department of the British Museum. 



of the folds dipping far down into 



the thecal cavity. One infers from similar foldings in other animals 



Theca. 



Brachioles. 



Root. 



Fio. IV. 



