546 F. A. Bather — Studies in Eclriomteroidea. 



internal cast by a knob. Of these pores from tbree to six may be 

 counted in 5 mm., according to the distance from the ambulacral 

 centre (2). (Figs. 2e,/.) 



A flattened, elevated, roughly pentagonal area over the actinal 

 pole in 1, 3, and 5, may indicate that this was roofed in by covering- 

 plates. Such a structure may also be inferred from the evidence of 

 allied genera. The covering-plates that doubtless once roofed in 

 the rest of the grooves, were pi'obably removed before fossilization. 



The Interradial Areas of the upper surface form slightly concave 

 depressions between the radial grooves. They were covered with 

 polygonal plates (about 2"5 X 2 mm., or less), having their longer 

 diameters parallel to the grooves ; there is no evidence that these 

 plates overlapped each other. 



In the interradius which (for that reason) has been here termed 

 ' posterior,' lay the anal opening, surrounded apparently by minute 

 plates, the number of which cannot be ascertained. The representa- 

 tion in Diagram 1 is purely diagrammatic. The rounded anal 

 eminence can be recognized on all the casts, at a varying distance 

 from the actinal centre and from the periphery. This interradius 

 is wider than the others near the actinal pole, and in that region 

 probably lay the hydropore. The interradial areas are really con- 

 tinuous with the test of the under surface ; but practicallj^ they are 

 separated, partly b}^ the almost complete encircling of the periphery 

 by the radial grooves, partly by a rather sudden change in the 

 structure of the test. 



The Under, or Abactinal Surface, was divided into two 

 areas, an inner or central, and an outer or peripheral, by a circular 

 Frame, corresponding in position to the rim of a Tam-o'-Shanter, but 

 turned inwards and slightly upwards, not downwards (Diagram 2). 

 Evidence for this frame is presented by specimens 2, 4, 3, and 7, 

 especially the two latter, which had this region protected by matrix. 

 Eemoval of the matrix, or cutting across it (Fig. 7^), shows a distinct 

 space, triangular in section, encircling the central area. This can 

 only be accounted for hy supposing that thicker calcareous plates 

 existed here, and have been dissolved away (cf. Figs. 7c, Id). 



Peripheral Area. — The frame was depressed to about half the 

 total thickness of the animal, and between it and the periphery there 

 rolled convexly a somewhat flexible integument in which were set 

 minute narrow plates, with their long axes at right angles to the 

 radii of the circle. The edges of these plates appear to have pro- 

 jected outward, towards the periphery of the test, so as to produce 

 an imbrication (1, 2, 3, 4, 7). Flexibility is ascribed to this area 

 chiefly on the evidence of 2, which shows a radial folding in places. 



Central Area. — What was inside the frame cannot be determined 

 satisfactorily. There are in 2, 3, and possibly in others, suggestions 

 of a fine and flexible membrane stretched loosely across the opening 

 of the frame. Traces of this are only seen near the edge, and there 

 is no proof that it was continuous. In 2, however, the visible traces 

 show a fine radial pleating, such as might have been produced, had 

 the membrane been continuous, by some pressure in the central 

 region. (Fig. 26.) 



