124 Br. F. A. Bather — Studies in Edrioasteroidea. 



appearances altogether, and that pores do not really exist (e.g. Jaekel, 

 1899, p. 21). The presence of pores, however, is proved by the more 

 direct evidence of sections fortunately provided by the cracks across 

 specimen A. One of these cracks (PL X, Fig. 1) passes right through 

 a supposed pore on the left side of the anterior ray ; removing tlie 

 loose portion and examining the fractured surface, one sees the dark 

 stain, of the pore passing right through the test to the dark impure 

 limestone that fills the thecal cavity (PI. X, Fig. 6). This section 

 also shows the oblique suture between the floor-plate and the adjacent 

 iuterradial, sloping from the exterior inwards towards the perradius ; 

 and the pore-canal is seen to have a similar obliquity. A like 

 appearance is seen at the distal fractured end of the right anterior ray, 

 which underlies the broken end of the anterior ray ; also at the crack 

 that traverses the same ray at right angles to the main crack. 



These pores differ from the so-called pores of the Cystidea in two 

 respects: they pass into the thecal cavity; they do not pierce tlie 

 substance of the floor-plates, but lie in the suture between them. 

 Just as the depression of that suture on the floor of the radial groove 

 forms a channel leading to the peripodium or circular rim round the 

 pore, so on the interior of the test the suture between the floor-plates 

 is depressed as it nears the outer margin of the ray, and thus the 

 pore-canal opens into an elongate depression, as well seen in a part 

 of the right anterior ray of specimen E 16172 (PL XIV, Fig. 2). 

 These depressions are the more conspicuous because the general inner 

 sui'face of the floor-plates is flush with the inner surface of the 

 interradials. These interior openings of the pores were noted by 

 E. Billings in 1854 (p. 272). 



It is clear that the appearances in the stereom of the floor-plates, 

 as just described, form the precise counterpart of the appearances in 

 the matrix and internal casts of Edrioaster huchianus as drawn and 

 interpreted in Study II (Geol. Mag., 1900, p. 196, Text-fig. 3). 

 What all these appearances may mean will be discussed later, but the 

 objective reality of the pores in Edrioaster will, I trust, no longer be 

 called in question. 



Certain other markings near the outer margin of the radial groove 

 will be described in connexion with the cover-plates, to which we 

 now pass, leaving the circumoral modifications of the floor-plates for 

 future consideration. 



EXPLANATION OF PLATES X-XII. 



Plate X. 



Edrioaster bigsbyi. Specimen A, from drawings made in 1899 by Mr. Gilbert C. 

 Chubb. 



Fig. 1. Adoral face. Towards the observer is the posterior interradius, 

 in which are seen the anus and the hydropore. The dark triangular 

 depression between those structures is due to fracture of the plates. 

 X 2 diam. 



Fig. 2. Posterior view, x 2 diam. 



Fig. 3. Adapieal face, the posterior interradius being away from the 

 observer. X 2 diam. 



Fig. 4. Some of the fioor-plates, showing the groove that leads from 

 the poral depression to the main radial channeL The depression for the 

 cover-plates is not clear in this lighting, x 10 diam. 



