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 

 crest (" oral ridge," Etheridge & Car- Radiala i -m m?/m 

 penter). Between adjacent A, and \lf_mr 



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- 

 aggeration of the axial folds SO con- 



flnirmmq in Asternhln<itiix tliA istprpmr. mis > from Kinderliook beds of Iowa. By 

 n ASteroblastllS , the Stereom permission of the Keeper of the Geological 

 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 



Brachioles. 



Theca. 



-Stem. 



Root. 



Fio. IV. 



Reconstruction of a typical Eublastoid, 



