1915 8 F. A. Bather — Studies in Edrioasteroidea. 



is confirmed by the position of some of the neighbouring spines. 

 Such tubercles were no doubt present on all the similar plates of this 

 and of the other grooves, but of those other grooves it is only the left 

 posterior which retains at all obvious traces of them ; on the exposed 

 portions of the remaining grooves they seem to have been less 

 protected by preserved spines, and so to have been worn away. 



The homologies of the alternating plates of the grooves are not 

 quite certain. Presumably they must be either floor-plates or cover- 

 plates. Since it is plain that they bore spines, they must have been, 

 at least in part, on the exterior; and one would naturally infer that 

 they were cover-plates. But they seem thicker and more curved and 

 rounded than the cover-plates in Edrioasteridse ; and they might 

 conceivably be floor-plates of which the elevated portions were 

 exposed, leaving only the edges, especially near the median line, to 

 be protected by small cover-plates now removed or unrecognizable. 

 Lebetodiscus (Study III, December, 1908) presents such a structure, 

 but in L. dichoni the pores between the floor-plates are clear, whereas 

 there is no trace of pores in Pyrgocystis sardesoni. 



The Interradiai Plates of the oral face are best exposed in the left 

 anterior interradius, and to a less extent in the right anterior inter- 

 radius. They merge into the imbricating plates of the turret, and 

 the appearance is as though the turret-plates gradually became 

 vertical and then inclined towards the oral centre. At the same time 

 they become slightly thicker and much narrower, so that their relative 

 thickness is considerably increased. 



In part of the right anterior interradius and in the three other 

 interradii these plates are obscured by Spines. Since these spines 

 are of the same character as those on the grooves, it may be inferred 

 that they were borne b} r similar tubercles. In the exposed interradii, 

 however, such tubercles cannot be distinguished among the numerous 

 irregularities of the surface. In the left posterior interradius some 

 tubercles seem to be exposed, but it is difficult to distinguish between 

 them and the ends of prostrate spines. 



In the posterior interradius, as already stated, several spines, 

 perhaps rather longer than most, are all directed in one way, and 

 some of these converge to a point on the margin, to the right of the 

 posterior interradius. A similar disposition of spines to protect the 

 anal opening is common in spiniferous echinoderms. 



The spines attained, in some cases at any rate, a length of not less 

 than 1-3 mm. and a diameter of -12 mm., tapering gently near the 

 distal end. They show no sign of longitudinal striation, but are 

 often irregularly blotched with black, which represents carbonized 

 stroma and indicates a relatively wide-meshed stereom. 



The Turret on which the oral face is supported has in specimen A 

 (PI. II, Fig. 4) a height of 132 mm., and it is uncertain whether 

 the actual base is preserved ; the cross-section is somewhat elliptical, 

 with diameters of 10 8 mm. and 8*8 mm. In specimen B (PI. II, 

 Fig. 6), which seems to be still attached to a piece of rock, the 

 height is 10 mm., and the diameters about 11 and 10 mm.; but here 

 some of the adoral end may be missing. Specimen C (PI. II, Fig. 3) 

 has a height of about 11 5 mm., with diameters 10 8 and 8-2 mm. ; and 



