356 ROBERT TRACY JACKSON ON ECHINI. 



Springer. It is named for the late- Professor James Hall, who did such extensive and valuable 

 work for Palaeontology in America. This species is near to Oligoporus danae, but differs from 

 it in that it has seven or eight instead of nine columns of plates in an interambulacral area; 

 also in that the interambulacra have a much less accelerated development than in danae. 



Keokuk Group, Lower Carboniferous, Boonville, Missouri, F. Springer Collection 8,114, 

 originally in G. Hambach's collection. 



Oligoporus danae (Meek an<l Worthen). 

 Text-figs. 12, p. 54; 237, p. 231 ; Plate 47, fig. 13; Plate 49, figs. 4, 5; Plate 50, figs. 1-12. 



Melonites danae Meek and Worthen, 1860, p. 397. 



Oligoporus danae Meek and Worthen, 1860a, p. 472; 1866, pp. 248, 249-251, te.\t-fig. 28, Plate 17, fig. 8; 



Loven, 1874, p. 42; Keyes, 1894, p. 126, Plate 17, figs. 2a, 2b; 1895, p. 182, Plate 20, figs. 2a, 2b; 



Jackson, 1896, p. 191, te.\t-fig. 1, p. 193, Plate 6, figs. 30-34; (pars) Kleni, 1904, p. 37; Lambert and 



Thiery, 1910, p. 121. 

 MeUmopsis [danae] Meek and Worthen, 1866, p. 249. 

 Palaechinus danae Meek and Worthen, 1866, p. 251. 



Test high, spheroidal, with moderately elevated melon-like ribs in ambulacral and inter- 

 ambulacral areas. Meek and Worthen say of the type that it is too imperfect to give any exact 

 measurements, but it indicates a height of about 4 inches and a breadth of probably from 3.5 

 to 4 inches. In the large specimen (Plate 49, figs. 4, 5), which is the most nearly complete 

 one known, the ambulacra measure 21 mm. in width at the mid-zone, the interambulacra about 

 45 mm. in width. The specimen, if complete, would measure about 115 mm. in height, and 

 about 120 mm. in diameter; it is therefore a very large individual. 



The ambulacra at the mid-zone are wide, composed of four columns of narrower demi- 

 and wider occluded plates with scattered isolated plates in the middle line of each half-area. 

 The occluded plates in the middle line are arched up as low, rounded, melon-like ribs. Pore- 

 pairs are in shallow valleys on either side, and situated in the outer border of each ambulacral 

 plate. Three or four demi-plates are equal in height to an adradial. Laterally, ambulacral 

 plates bevel over the interambulacrals on the adradial suture. Ventrally, at the peristomal 

 border, there are a few primary ambulacral plates that cross the half-area, and the pore-pairs 

 are uniserial (Plate 50, fig. 8) . This is like the character of Palaeechinus at the mid-zone, and 

 like Maccoya and Lovenechinus at the ventral border. Higher up on the right side of area 

 J are two plates which are alternately primary, expanded on the outer border, and occluded, 

 being cut off from interambulacral contact by the enlargement of its fellow. This stage, 

 though very abbreviated, is like the character of Maccoya at the mid-zone, and like the second 

 stage in the development of Lovenechinus. Higher again, there are four columns of plates, 

 demi- and occluded, but without any isolated plates in the middle of the half-areas. This 



