402 ROBERT TRACY JACKSON ON ECHINI. 



one in the Griffith Collection in Dublin. Of these, the Cambridge specimen is apparently the 

 one from which M'Coy's figures were taken, and is the original of Keeping's (1876) descrip- 

 tion and figures here reproduced as Plate 64, figs. 7, 8. M'Coy's second specimen is doubtless 

 the one in the Griffith Collection in Dublin, here reproduced as Plate 29, fig. 5. 



The test is spheroidal, and representative measurements are as follows. The Griffith 

 specimen (Plate 29, fig. 5) measures 55 mm. in diameter through the plane J, E, and the inter- 

 ambulacra at the periphery, which is about at the mid-zone, measure 28 mm. in width. On 

 account of crushing, the width of the ambulacra could not be ascertained. The Trinity College 

 specimen, dorsal view (Plate 62, fig. 7), measures about 52 mm. in diameter. The ambulacra 

 at the adoral margin, which is about the mid-zone, measure about 3 mm. in width, and the 

 interambulacra about 25 mm. in width. The Trinity College specimen, internal view (Plate 62, 

 fig. 6), measures about 50 mm. in diameter; the ambulacra at the aboral border, which is about 

 the mid-zone, measure 8.5 mm., and the interambulacra about 19 mm. in width. It is seen 

 from this that the ambulacra on the interior are more than twice as wide as on the exterior. 

 This is due to the strong lateral beveling of ambulacral plates under the adradials. Con- 

 currently the interambulacra are narrower than on the exterior, due largely to the same cause 

 (compare text-figs. 32, 34, p. 75). 



The ambulacra are narrow, especially dorsally, and are composed of two columns of low 

 plates which imbricate strongly adorally, and laterally bevel strongly under the adradials. 

 Dorsally, all plates are alike, typical primaries with pore-pairs uniserial (Plate 64, fig. 5), but at 

 the mid-zone alternate plates tend to be narrowed laterally, and may in cases even be cut off 

 from contact with the interambulacra. Pore-pairs are situated in the outer portion of each 

 plate, and are dorsally uniserial, but at the mid-zone are somewhat irregularly slightly biserial. 

 On the interior (Plate 64, fig. 2) ambulacral plates are much wider than on the exterior on account 

 of the lateral beveling, and the pore-pairs are near the middle of each plate. The interambu- 

 lacra are wide, composed at the mid-zone of five columns of plates in each area. The plates 

 are relatively thick, and imbricate strongly aborally and from the center laterally and over the 

 ambulacrals on the adradial suture. The plates are curved outward on the external surface 

 and rounded on the suture lines. The adambulacral plates are pentagonal, strongly rounded 

 on the adradial sutures. Plates of median columns are rounded polygonal. Each adradial 

 plate bears an eccentric perforate primary tubercle with scrobicule and, in addition, secondary 

 tubercles. Plates of median columns may have a primary tubercle with secondaries, or appar- 

 ently the latter only. Dorsally, in each interambulacrum in the known specimens two columns 

 drop out before reaching the apical disc. Ventrally, the primordial interambulacral plates 

 are in place in the basicoronal row (text-fig. 30, p. 70; Plate 64, fig. 2). There are two plates 

 in the second row, three in the third, four in the fourth, and five in the fifth row, above which 

 no more columns are added. Primary spines are expanded at the base, tapering, and about 

 12 mm. long. Secondary spines are similar, but fine and slender, and about 3.5 mm. long. 



