392 ROBERT TRACY JACKSON ON ECHINI. 



been attained at the mid-zone, somewhat earlier than in the type. A fragmentary specimen 

 in the United States National Museum has eleven columns at a point which is probably 

 very near to the mid-zone (Plate 61, fig. 8). 



Dorsally, there are obscure traces of genital and ocular plates in the type, but structural 

 details could not be made out. As closely as it can be ascertained the apical disc measures 

 about 22 to 25 mm. in diameter, which is 15 to 16 % of the diameter of the test. In this 

 proportion it agrees with the measurements obtained in Melonechinus multiporus (p. 379). 



This species is very distinct from any previously described. It differs from multiporus 

 in the character of the highly elevated melon-like ribs, in the number of columns of ambulacral 

 and interambulacral plates, and the accelerated development of the interambulacral area. 

 I think Miss Klem is entirely mistaken in considering giganteus a synonym of multiporus, but 

 it should be noticed that she includes in the synonymy of multiporus several species that are 

 here considered distinct. Miss Wood (1909, p. 108) thinks that the Melonites granulatus 

 of Troost, is referable to giganteus, and studying the type, I quite agree with her, although my 

 first impression was that it should be considered a distinct species. In this specimen (Plate 58, 

 fig. 2; Plate 59, fig. 4), which is fragmentary, there is a half-ambulacrum with six columns of 

 plates, indicating twelve columns in the area. In the interambulacrum there are an adambula- 

 cral and five hexagonal columns in place, but the right part of the area is wanting, so that 

 the total number of interambulacral columns is unknown. With present evidence it can safely 

 be referred to giganteus. 



Lower Carboniferous, Bowling Green, Kentucky, holotype, Museum of Comparative 

 Zoology Collection 2,989; St. Louis Group, Clarksville, Tennessee, United States National 

 Museum 42,340; Mississippi Group, Lower Carboniferous, Cumberland Gap, Tennessee (the 

 holotype of Melonites granulatus Troost) United States National Museum 39,909; no detailed 

 horizon or locality, probably St. Louis Group, Tennessee, Vanderbilt University Collection 

 (Nashville, Tennessee), 222. 



The several specimens of this species studied are sufficiently considered above, excepting 

 the National Museum specimen from Clarksville, Tennessee. This specimen, though fragmen- 

 tary, is very instructive. From it is gathered a very good section across an interambulacrum 

 and half -ambulacrum (Plate 61, fig. 8). In the ambulacrum there are six columns of plates in 

 the half-area, pore-pairs are in clear peripodia near the outer border of each plate, and numer- 

 ous secondary tubercles exist. The interambulacrum shows the introduction of column 11 

 in a pentagonal plate with a heptagon on its right ventral border. A section of an ambulacrum 

 seen from within (Plate 61, fig. 9) shows plates more angular than on the exterior and convex 

 toward the interior. The pore-pairs lie toward the outer border of the occluded plates, the 

 inner border of demi-plates, and near the middle of isolated plates, instead of near the outer 

 border of all plates, as seen on the exterior. In order to compare critically the inner and outer 

 faces, these views are shown in the same individual plates in Plate 61, figs. 5, 6, and 7. In these 



