IV. Trenton Limestone Edrioasters. 1914 166 



distinct interradial elements, but the appearance of such in external 

 view, as described above, is almost certainly due to the overlapping 

 of the perradiad ends of the proximal floor-plates. The difficulties of 

 interpretation are due to the very close union of the plates to form 

 a rigid mouth-frame, and to the cracks that develop during the 

 process of preparation. 



There appears to be in each ray some slight projection of the 

 interporal region of a pair of floor-plates, each forming a process 

 directed away from the perradius, and placed at the spring of the 

 elevated border. These are not so regular that one can infer their 

 normal and constant presence in every ray of all individuals, but they 

 may well have some significance as processes for the attachment of 

 muscles or other internal organs. 



There are no traces of any other hard mouth-parts or of any skeletal 

 connexion between the mouth-frame and the lobes of the adapical 

 integument. 



These peristomial structures are the stereom counterpart of the 

 various channels and cavities observed in the internal cast of Edrioaster 

 buchianus, and confirm the interpretation of those cavities offered in 

 !Study II, pp. 197, 198. Their relations to the soft parts will be 

 discussed later. 



The Interradial Areas are irregular in shape according to the 

 varying curves of the radial grooves. In A, the posterior area, from 

 the concavity of the left posterior ray to that of the right posterior, 

 is 19*5 mm. across ; but a part of this is occupied by the recurved end 

 of the right posterior groove. Of the other areas, the widest is the 

 left anterior, which measures 13 mm. at its widest part; and the 

 narrowest is the right posterior, which measures about 7*5 mm. near 

 the periphery (PI. X, Fig. 1). In B the same general relations 

 obtain (PL XI, Fig. 1). In E 15930 the chief difference seems to be 

 that the left anterior area is relatively wider. In E. levis, on the 

 other hand, the relations, owing to the solar curvature of all the rays, 

 are quite different ; thus, in E 15900 the measurements are : posterior, 

 circa 24 mm.; left and right anterior, both 12 mm. ; right posterior, 

 circa 16 mm. (Plate XII). 



The Interradial plates are irregular polygons, apparently with no 

 definite arrangement. A single plate of moderate size abuts on the 

 compound interradial element of the peristomial frame and is usually 

 bordered by about four floor-plates on each side. This is followed by 

 a number of plates different in each interradius. Thus in A the 

 numbers are: 1. post., 2; 1. ant., 3; r. ant., 4 or 5 ? ; r. post., 4. 

 In B these plates almost agree in numbers and arrangement with 

 those of A. In these two individuals the curvature of the rays is 

 almost exactly the same, and there is also a remarkable general 

 similarity in the number, shapes, and arrangement of the interradial 

 plates. This similarity may imply either that the plates are heritable 

 morphological elements, or that similar mechanical causes acting upon 

 an indifferent plate-forming tissue have produced a similar breaking-up 

 of the stereom. That the latter is the true interpretation seems to be 

 proved by the other specimens, for in them the same plates cannot 

 be identified. 



