322 ROBERT TRACY JACKSON ON ECHINI. 



that point, the plates between being narrowed marginally, occluded, and cut out from interam- 

 bulacral contact by the enlargement of their fellows; pore-pairs biserial (Plate 47, fig. 2). This 

 is the typical generic character. On the interior of the test all ambulacral plates are of about 

 equal height and cross the half-area completely, as shown in a specimen from Rahan's Bay, 

 in the Sedgwick Museum, Cambridge, England, no. 3, which also shows that the ambulacral 

 plates bevel over the adambulacrals on the marginal suture, a structure similar to that shown 

 in Maccoya intermedia (Plate 34, figs. 2, 3). 



The interambulacra of the type (Plate 46, figs. 1, 2) are more nearly complete than those 

 of any other known specimen of the species. This specimen is bent sharply on itself, as on a 

 hinge, so that on one side (Plate 46, fig. 1) is seen an ambulacrum and three adjacent inter- 

 ambulacral columns, and on the reverse side (Plate 46, fig. 2) is seen an adradial pentagonal 

 column and an adjacent column of hexagonal interambulacral plates. This structure is shown 

 clearly in Baily's (1874, Plate 3, figs, a, b) excellent figures. M'Coy in his restoration assumed 

 six columns in an area, which is very likely correct, but there maj' have been fewer, or possibly 

 more than that number. The plates are very large and relatively high in proportion to their 

 width. The pentagonal adambulacral plates measure about 12 mm. in width by 9 mm. in 

 height; the hexagonal plates measure about 14 mm. in width by 9 mm. in height. These plates 

 bear numerous secondary tubercles, as shown better ;n M'Coy's second type (Plate 46, fig. 3). 

 The characters of the ventral and apical areas are unknown. This species is the type of the 

 genus. 



Arenaceous shale, Lower Carboniferous, Rahan's Bay, Dunkineely, County Donegal, 

 Ireland, cotypes, two specimens, in the Griffith Collection, Science and Arts Museum, Dublin; 

 same locality, Sedgwick Museum, Cambridge, England, Collection 3 and 266; Lower Carbonif- 

 erous shale, Drumlug, County Donegal, Ireland, Trinitj' College Museum, Dublin; Regny, 

 Central France (Julien). 



The specimen from Drumlug in the Trinity College Museum shows an ambulacrum very 

 clearly, in which all plates meet the middle of the area, but alternate plates are primary and 

 occluded, meeting or being cut off from the interambulacrum ; pore-pairs are biserial, as in 

 M'Coy's type. There is a column of large pentagonal adradial plates in place with the ambu- 

 lacrum and in addition scattered interambulacral plates. One large adradial plate measures 

 16 mm. in width and 10 mm. in height. 



The specimen in the Sedgwick Museum Collection no. 3, is an interesting individual of 

 this species from Rahan's Bay, the type locality. This is the original of Keeping's (1876) Plate 

 3, fig. 13, as stated on the old and somewhat faded label. His figure, however, is somewhat 

 inadequate. The specimen is a slab on which there are portions of an ambulacrum and ten 

 more or less perfect and dissociated interambulacral plates. The ambulacral plates are in 

 three short, but continuous pieces. All plates meet the middle of the area, but alternate plates 



