1914 165 F. A. Bather — Studies in Edrioasteroidea. 



element, over which the groove itself passes centripetally down into 

 the thecal cavity. As seen from above each of these radial flooring 

 elements has a roughly triangular outline, and meets its fellows 

 to left and right by its basal angles, thus forming a continuous ring 

 round the central space. In 1854 (p. 271) E. Billings figured these 

 plates as seen from the interior (fig. 11) and in section (fig. 12). He 

 described them as triangular, and partly covered by the plates at the 

 interradial angles, and as having "an elevated border on the side 

 next the mouth, below, caused by the bending down of the plate." 



To obtain a clearer idea of the relations of these apparent radial 

 mouth-plates, I have removed the adapical structures and the matrix 

 from the under side of rays III and IV in the peristomial region of 

 E 16172. After three months of this labour it has been possible to 

 produce Fig. 2 of Plate XIV, but the structures are still rather 



floor-plate 1. post, ray 



accessory hydropore radial interradial 1. ant. ray 



cover-plate peristomial 

 element 



Edkioaster bigsbyi. Specimen A. 

 Fig. 3. Portion of the peristomial region more developed by preparation than 

 in Plate X, fig. 1. x 4 diam. 

 ,, 4. The distal end of the left anterior ray, showing floor-plates only, 

 x 4 diam. 



difficult of interpretation. The peristome, as viewed from the inside 

 of the test, is surrounded by a stout frame forming an elevated border. 

 This border is flattened rather curiously, and is widest on the perradii, 

 so that while its inner or adcentral margin is approximately circular, 

 its outer or adperipheral margin approaches a pentagon with radial 

 angles. The interradial tracts of this mouth-frame are, it is clear, 

 composed of those floor-plates which, as already described, are fused 

 to form the adoral or proximal interradial plates. The pores between 

 these plates penetrate to the interior all the way round, though they 

 decrease in size as they near the interradius. The radial tracts of 

 the frame likewise seem to be composed of the floor-plates, which are 

 here enlarged at their perradiad ends. There do not seem to be any 



