V. Steganoblastus. 1914 197 



plate, continuous with the adambulacral series, and abutting above on 

 the corresponding oro-tegrainal. In the posterior interradius this 

 plate occupies a larger area than in the others, and is much more 

 swollen. Close examination under suitable illumination detects in 

 this plate, in all three specimens, a meridional dark line. This is not 

 a suture, for it is waved and does not reach the edges of the plate. 

 It may represent the Hydropore. 



The Ornament of the adapical system consists of the axial folds 

 already mentioned, and of a number of pits. 



The Axial Folds in the lower, proximal or adcolumnal region of 

 the theca are fairly regular. From the lip of each radial there run 

 four main ridges, one pair horizontally to the lips of the adjacent 

 radials, one pair downwards to the middle of the base of the adjacent 

 basals. These main ridges thus enclose triangular spaces, namely an 

 almost equilateral triangle bounded above by the horizontal ridges, 

 and interradial in position ; an acute-angled triangle with its base on 

 the columnar suture, and radial in position. The interradial triangle 

 encloses a smaller, less regular triangle of ridges. The radial triangle 

 encloses two shorter ridges also springing from the base and meeting 

 at a wider angle (circa 90°) at about one-third the height of the 

 radial triangle itself. The triangular interambulacral area, above 

 the main horizontal ridge, contains ridges crossing the sutures of the 

 various plates already described as interambulacrals and adambulacrals. 



The folds between the ridges are very deep, especially in the 

 interambulacral area, notably a fold just below the adoral inter- 

 ambulacral (Text-fig. 3). The removal of the matrix from these is 

 extraordinarily difficult, and has therefore only been accomplished in 

 one or two places. The operation led to no discovery of pores or any 

 system of respiratory folds, and it may be inferred that the sole effect 

 of the folding is to endue the theca with a quite exceptional rigidity. 



The Pits cover the whole outer surface of the theca, including all 

 the elements of the adoral system (PI. XV, Figs. 2, 3). They have in 

 many cases been worn away from the more prominent parts, either 

 in the process of cleaning or, as there is some evidence to show, 

 before the fossil was embedded in the matrix. They seem to lie even 

 in the depressions of the folds, although not quite so well marked 

 there. The pits seem to have been set most closely on the cover- 

 plates and tegmen, and in consequence may there assume a more 

 hexagonal outline. Elsewhere they are more circular. They vary 

 in size, but have as a rule a diameter of about '2 mm. Though they 

 have no definite arrangement, they tend to lie along the edges of the 

 cover-plates and along the sides of the larger folds. Occasionally 

 there is a tendency to similar seriation at a definite angle to the sides 

 of the grooves, as though marking the original adambulacral plates. 

 These structures are pits, not pores, for one cannot imagine pores on 

 cover-plates, and in any case they disappear entirely when worn 

 down. No definite structure can be distinguished in them, but their 

 complete resemblance to the spine-pits of Asteroidea and the lack of any 

 other explanation render it highly probable that they lodged spines. 



The Subvective System consists of five broad radial grooves leading 

 to a subtegminal mouth. The grooves are bounded by Floor-plates, 



E 



