II Edrioaster Buchianus. 



VIT. 197 



(certainly more than the 50 which Forbes counted). The thickness 

 of the plates towards their outer margins was about 1 mm. The 

 plates were continuous with the interradial thecal plates, but rather 

 thicker ; at a little distance from the edge, they bent downwards 

 at a rather sharp angle, so as to form a V-shaped trench with 

 slightly convex sides. Seen from the inside the plates formed 

 a ridge, with sides at first steep, then gradually rounding over. 



Fig. 4. — Reconstructed section across a subvective groove, x 6 diam. Compare 

 with Fig. 1 on p. 195, and Fig. 3 on p. 196. The relations of the covering- 

 plates are inferred from Edrioaster higsbyi. The position of the perradial 

 water- vessel and its branch to the podium is deduced from the remains of 

 E. Bigsbyx and the present species. Natural suture -surfaces are dotted ; cut 

 surfaces are ruled diagonally ; supposed soft parts are in broken outline. 

 v.g. subvective groove, c.p. covering-plates, f.p. flooring-plates, i.a. 

 interambulacrals. w. perradial water-vessel, connected by a branch with the 

 podium, p. amp. ampulla connected with the podium through the pore. 



The adjacent sides of the plates were excavate so that pores were 

 formed between them. Thus the groove was fringed with a row 

 of pores on either side. These pores lay, not at the extreme 

 margin, but just where the plates bent downwards ; they passed 

 outwards at an angle sloped away from the actinal centre ; they 

 were wider in the proximal portion of the groove, and more elongate 

 in the distal portion. We may infer from the as yet undescribed 

 structure of Edrioaster Bigsbyi, that the groove was roofed by 

 covering-plates, which rested on the flattened border. But these 

 covering-plates must have been detached and washed away before 

 the theca was imbedded, since no traces of them remain. 



Around the Actinal Pole the form of the internal cast is rather 

 complicated (PI. X, Fig. 2). The grooves curve down towards the 

 pole, and at 2 or 3 mm. from the pole itself they bend downwards 

 sharply. The central region is occupied by an irregularly shaped 

 mass of matrix, with a broken surface about 3*5 mm. below the highest 

 point of the cast. This mass probably represents the oesophagus. 

 It is surrounded by a channel or, as it were, a moat, into which the 

 down-bent grooves lead. At certain points, however, this moat is 

 bridged by matrix, and in the two rays of the left side the extreme 

 proximal ends of the grooves are similarly bridged. These bridges 

 look like continuations of the prominences or infillings of the proximal 

 pores. In the posterior interradius is a similar lump of matrix, 

 mistakenly alluded to by Forbes (op. cit., p. 522) as " the projection 

 which bore the anus." This is separated from the oesophageal 



