376 ROBERT TRACY JACKSON ON ECHINI. 



ing the choice specimens in Mr. Frank Springer's collection recently acquired by him from 

 Professor Hambach of St. Louis. In my earlier paper with Professor Jaggar (1896) we pub- 

 lished a detailed description and discussion of this species, so that here I shall give onlj' the 

 systematic description of the species, followed by a discussion of new observations. 



The test is high and spheroidal, with low, rounded, melon-like ribs in ambulacral and 

 interambulacral areas. A small but nearly perfect specimen, free from all distortion (Plate 55, 

 figs. 1, 2), measures 54 mm. in height and 62 mm. in diameter through the mid-zone; the 

 ambulacra at the mid-zone measure 16 mm. in width, the interambulacra 20 mm. in width. 

 A larger specimen, also quite free from distortion (Plate 60, fig. 1), measures 65 mm. in height, 

 79 mm. in diameter through the mid-zone; the ambulacra at the mid-zone measure 20 mm. in 

 width and the interambulacra 25 mm. in width. A very large specimen, the largest that I 

 have seen (Plate 55, fig. 3), is incomplete ventrally, and the figure shows practically the whole 

 specimen. It measures 123 mm. in diameter through the mid-zone. Comparing this with 

 smaller specimens, the estimated height is about 101 mm.; the width of the ambulacra at 

 the mid-zone is 34 mm., and of the interambulacra 39 mm. 



The ambulacra are somewhat narrower than the interambulacra, with ten columns of 

 plates at the mid-zone (Plate 56, fig. 4). These consist of wide occluded plates, extending 

 from the middle of the area outward, narrow demi-plates extending from the interambulacral 

 contact inwax'd, and three irregular columns of isolated plates in each half-area. Occasionallj', 

 only eight columns of ambulacral plates can be counted at some planes, but the character is 

 to have ten columns. The occluded plates in the median line are elevated in low, gently 

 rounded, melon-like ribs, and demi-plates bevel over the interambulacrals on the adradial 

 sutures, as I showed previously (1896, p. 141, Plate 2, fig. 5). The pore-pairs are situated in 

 shallow valleys on either side of the ambulacral areas and lie in the outer portion of each plate, 

 the pores being surrounded by a peripodium (Plate 56, fig. 12) ; but this structure is usually 

 lost in erosion. Ambulacral plates bear small secondary tubercles and spines, similar to those 

 of the interambulacral areas. As seen from the interior (Plate 56, fig. 5), the occluded plates 

 are narrower than on the exterior, and demi-plates opposite horizontal interambulacral sutures 

 are higher and laterally fan-shaped (compare text-fig. 244, p. 338). The pore-pairs on the 

 interior of the test lie near the outer border of the occluded plates, the inner border of the demi- 

 plates, and near the center of isolated plates, instead of all near the outer border of plates 

 as seen from the exterior of the test (Plate 56, figs. 4, 5), (p. 61). 



Ventrally, near the peristomal border the ambulacra have four columns of plates (Plate 56, 

 figs. 2, 3; Plate 57, fig. 1), and the complex character of the ambulacrum is built up passing 

 dorsally, as later described in detail. Dorsally, near the oculars in the placogenous zone (Plate 

 56, fig. 6) a simpler condition also exists as a localized stage, and, passing ventrally to earlier 

 built plates, the complex character is rapidly built up, as discussed later in detail. 



