200 PROCEEDINGS OF THE ACADEMY OF [1881. 



in the centre, and following the median line of the radial depres- 

 sions and galleries below the vault to the arm openings. 



In some specimens of ActinoorinidjB, almost the entire test is 

 lined with a delicate calcareous i)lexus or network. This lining 

 is not in contact with the test directly, but connected with it by 

 small partitions, producing innumerable little chambers, which 

 eommunicate with each other and with the visceral cavity. 

 Thi're is, at least, one such partition or support from each plate 

 of the test, generally arising from the median portion of the plate. 

 (PI. XIX, fig. 16). The plexus is very delicate in some specimens, 

 in others — mostly in large specimens — rather dense and rigid, 

 but in all of them perforated with conspicuous pores or passages, 

 whose arrangement corresponds with the direction of the 

 sutures between the plates of the test. There is one pore at 

 least to each angle of the plate, but sometimes additional ones 

 in large individuals. The structure extends but little below 

 the regions of the second radials, leaves passages at the arm 

 openings, and tow^ard the vault reaches to a place near the 

 median portion of the ray, leaving at the centre an open space in 

 the test which is occupied by the central vault piece. From this 

 space five wide avenues, corresponding with the grooves on 

 the inner surface of the vault, pass out toward the arm bases. 

 The avenues produce five subtriangular interradial or interpalmar 

 fields, which are raised conspicuously above the floor of the vault. 

 They are of rather dense texture at the borders, the inner side 

 somewhat thickened, while the surface of the median portions is 

 rough and uneven, perforated with large and small passages which 

 communicate with the avenues. 



Four of the interpalmar fields are equal, the posterior one fre- 

 quently larger and penetrated by the anal aperture. In species 

 with a lateral opening all five fields have about the same form, 

 and the central space between them is of pentangular outline. The 

 case is different in species with a subcentral anus, when frequently 

 the posterior field is larger and encroaches deepl}^ upon the middle 

 space, giving to it a lunate instead of a subcircular or pentangular 

 outline. In species of this kind, the anus is placed near the edge 

 of the interpnlmar field, but separated from the central space 

 by a partition which forms the border of the field. In species 

 with secondary radials, the avenues divide, sending a branch to 



