MACCOYA. 315 



the interambulacral plates overlap from below upwards [imbricate aborally] and that the am- 

 bulacrals overlap in the opposite direction. Except for the bevel on the adradial suture I 

 have seen no imbrication of ambulacral or interambulacral plates in this or any other species 

 in this family. 



The apical disc is relatively small. Ocular plates may be insert, or in one specimen one 

 ocular is exsert, and in another all oculars are exsert (Plate 33, figs. 11, 12). It is very rare 

 for oculars to be exsert in the family (p. 89). The genitals are wide and high, with several up 

 to five genital pores each. The periproctal plates are unknown. This species seems to be 

 indistinguishable from M. burlingtonensis, but in view of the incomplete knowledge of the 

 structure and the wide geographical separation it seems best to maintain both species. 



Lower Carboniferous, Hook Head, County Wexford, Ireland, several specimens, including 

 the holotype, Sedgwick Museum, Cambridge, England; four specimens, Munich Museum; 

 two specimens. Trinity College, Dublin; Museum of Practical Geology, London, Collection 

 16,302; Museum of Comparative Zoology Collection 3,002. 



Of the Munich specimens of this species, one (Plate 33, figs. 6-9) has four columns of plates 

 in one interambulacral area, and also shows the tubercles clearly. The ambulacral detail is 

 clear on the surface with alternate primary and occluded plates, but in a portion showing the 

 interior the plates at this area are all primary in character with pore-pairs uniserial. Another 

 Munich specimen (Plate 34, fig. 1) is worn, but shows the form of the test quite well. There 

 are four columns of plates in two interambulacral areas. In area C in the middle line dorsally 

 there is in addition a single plate which may be considered a tendency toward an extra column 

 as a progressive variation. The only ocular preserved is insert. 



In the Sedgwick Museum, Cambridge, England, there are seven specimens, including 

 Keeping's holotype. Four of these have four columns of interambulacral plates in an area. 

 The type specimen (Plate 33, fig. 10, after Keeping) is most interesting. It has two columns 

 of interambulacral plates in place and displaced plates representing a third column. The 

 left adradial column is wanting in the more nearly complete area. The ambulacrum is slightly 

 pulled away from the adradial plates on the marginal suture, and reveals the structure graphi- 

 cally (Plate 34, figs. 2, 3). To understand it one must remember that ambulacral plates bevel 

 over the adradials, so that the marginal sutural face of the adradials is an inclined, not a vertical 

 face. On the exterior or distal aspect of this face four facets appear for the reception of the 

 primary ambulacral plates, and these facets in passing proximally become much narrower. 

 On the proximal face of the suture, between the narrowed facets, are seen other alternating 

 facets which receive the impact of the occluded ambulacral plates. This relation is perfectly 

 clear, for on the exterior of the specimen (Plate 34, fig. 2) ambulacral plates are alternately 

 primary and occluded, and pore-pairs biserial, whereas on the interior, as shown in part (Plate 

 34, fig. 3), ambulacral plates all cross the half-area and pore-pairs are uniserial. Because of 

 this structure it seems that from a single adradial plate one could determine the genus. 



