VI. Pyrgocystis. 



1915 11 



be of the same height and to reach approximately the same level. 

 This height is about 1*3 mm. as measured from the top of the turret. 

 The diameter of the cup at its upper margin is 4*7 mm., and below 

 is about 3-8 mm. 



The dome of the Oral Face reaches at its oral pole a height of 

 3'2 mm. above the cup, i.e. 4'5 mm. above the turret. 



A single Subvective Groove faces the observer, and others are 

 less clearly seen, one at each side. The groove attains a width of 

 1*7 mm., and is filled with alternating plates, which, as in 

 P. sardesoni, may be either cover-plates or floor-plates. These 

 plates are directed from the perradius outwards in a distal direction ; 

 the angle at which they meet is not easy to measure, but may be 

 estimated at about 133°. The distal end of the groove is pressed 

 against the plates of the cup, so that the arrangement of the 

 terminal groove-plates cannot be made out. There are about three 

 of these plates within a distance of 1 mm. A section across the 

 groove-structures would show a broad convex curve sinking rather 

 rapidly at each end towards the interradial area. The sutures 

 between the groove-plates are depressed, especially along the median 

 line of the groove, and appear rather irregular, as if they were either 

 slightly crenelate and interlocking, or marked with alternating 

 notches. There seems to be slight adoral imbrication of these plates. 



Towards the oral pole the groove narrows considerably, and the 

 curve of the cross-section is more markedly convex, almost angular. 

 At the oral pole the groove-plates appear to meet those of the 

 other rays, and there is no depression. That affords an argument 

 for the plates being cover-plates. 



The Interradial Plates of the oral face are hard to interpret. There 

 seems to have been a single large plate within, and as it were 

 continuing, the large overlapping plates of the cup. Then between 

 this and the adoral groove-plates seems to be a single small plate. 



The precise relation of these two interradial plates to the groove- 

 plates is not easy to make out. There are appearances which 

 remind one of the depressed sutures which in Steganoblastus continue 

 the line of the sutures between the cover-plates. It might be 

 inferred from this that the groove-plates, whether cover-plates or 

 floor-plates, were in places separated from the larger interradial plates 

 by small plates continuing the same line. 



The surface of all the plates of the oral face is slightly rough, but 

 it is not possible to say whether or no they bore spines. 



The Turret is distinctly separated from the cup, at least in this 

 unique specimen, owing to the fact that the cup-plates are more 

 vertical, or in other words more parallel to the long axis of the 

 animal, than are the uppermost turret-plates, and this divergence 

 of angle results in a groove where they meet. It is, no doubt, 

 conceivable that in other individuals, or even in this individual 

 under other conditions of muscle-tone in life or of preservation in 

 death, the uppermost turret-plates might be applied closely to the 

 plates of the cup ; but the distinction between cup and turret would 

 be none the less decided, by reason of the less height of the turret- 

 plates, or at any rate of their exposed portion. There is no reason 



