390 ROBERT TRACY JACKSON ON ECHINI. 







elevated melon-like ribs in ambulacral and interambulacral areas. The type specimen (Plate 



60, fig. 3) measures 115 mm. in height; the height through the exact center from the oral to 

 the anal area measures 100 mm. This smaller measurement is due to the ventral and dorsal 

 incurving of the test. The greatest diameter through the mid-zone is 155 mm. This measure- 

 ment is somewhat affected by dorso-ventral compression, which, while reducing the height, 

 exaggerates the horizontal diameter. Width of the ambulacra at the mid-zone 38 mm., width 

 of interambulacra 41 mm. The plates are very thick, the interambulacral plates at the mid- 

 zone measuring 8.5 mm. in thickness, much exceeding the surface width of the plates, which 

 is about 7 mm. In a fine specimen in Vanderbilt University Collection 222, the ambulacrum 

 measures about 40 mm. and the interambulacrum 45 mm. in width. 



Ambulacral areas are wide, with twelve columns of plates at the mid-zone composed of 

 wide occluded, narrow demi-, and four irregular columns of isolated plates in each half-area 

 (Plate 61, fig. 8). The occluded plates in the middle are arched up in high, rounded, melon- 

 like ribs formed by the thickening of the plates (Plate 60, fig. 3). Demi-plates bevel over 

 the adambulacrals on the adradial sutures. Pore-pairs are situated in sunken valleys in each 

 half-area; they are surrounded by peripodia and lie in the outer portion of each ambulacral 

 plate (Plate 61, fig. 8). Ambulacral plates are thickly covered with small secondary tubercles 

 like those of the interambulacra (Plate 60, fig. 3). As seen from the interior of the test (Plate 



61, figs. 6, 9), 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). In this view pore-pairs, as usual in the genus, lie in the outer portion of occluded 

 plates, the inner portion of demi-plates, and near the center of all isolated plates, instead of near 

 the outer border of all plates, as is the character of the exterior (Plate 61, figs. 5-9), (p. 359). 



Ventrally, the ambulacrum of M . giganteus presents interesting stages in development. 

 Near the peristomal border there are four columns of plates, narrower demi- and wider occluded 

 in each half-area, as shown in Plate 59, fig. 13. This is like the adult character of the genus 

 Lovenechinus (text-fig. 237, p. 231). A little higher up in this same figure single isolated plates 

 appear in the middle of each half-area; this is like the character of Oligoporus. Next appear 

 more isolated plates which, being continuous, make a single column of isolated plates in each 

 half-area like the condition in Melonechinus springeri, the simplest ambulacral condition that 

 is referable to the genus Melonechinus (text-fig. 237, p. 231). A four-column stage like that 

 of Melonechinus indianensis would be the next to follow the three column stage, but, owing 

 to local imperfections, it was not ascertained in giganteus, so that a gap occurs at this point. 

 A little higher up, however, we find a stage in which there are five columns in a half-area (Plate 

 59, fig. 12); this is like the typical condition at the mid-zone in Melonechinus multiporus (text- 

 fig. 237, p. 231). Still higher appear the six columns in a half-area, which is the species char- 

 acter. Thus the ventral stages in development of the ambulacrum in this highest species of 



