OLIGOPORUS. 351 



in Lovenechinus the highest known species (septies) has only seven columns in an area. Oligo- 

 porus differs from Melonechinus, as in that genus the isolated plates are sufficiently numerous 

 so that they form one or more irregular columns of plates between the demi- and occluded plates 

 in each half-area. Melonechinus is also more evolved in the interambulacrum in that the high- 

 est species has eleven columns of interambulacral plates, whereas the highest species of Oligo- 

 porus has but nine. In the development of the ambulacrum ventrally, Oligoporus passes 

 through a stage in which the plates are like Lovenechinus at the mid-zone; and Melonechinu- 

 in the development of its ambulacrum ventrally passes through stages in which the plates arc 

 successively like Lovenechinus, then like Oligoporus at the mid-zone (text-fig. 237, p. 231). 

 Thus both the structure in adults and the progressive stages in development unite these genera 

 into a phylogenetic whole. 



Key fit the SjH'rii'.t of 



Interambulacra with six or seven columns of plates in an area (in one area of one specimen, as an excep- 

 tional arrested radial variation, there are four columns only); melon-like ribs moderately developed 



(). hlairi Miller and Gurley, p. 351. 



Interambulacra with seven columns of plates in an area, or in part with six columns only; melon-like ribs 

 strongly elevated and rounded in outline ..... 0. corryi Meek and Worthen, p. 353. 



Interambulacra with seven columns of plates in an area; melon-like ribs strongly elevated and sharply 

 rounded ; interambulacra dipping steeply down to adradial sutures . 0. sulcatus Miller and Gurley, p. 354. 



Interambulacra with eight or seven columns of plates in an area . . . 0. halli sp. nov., p. 355. 



Interambulacra with nine columns of plates in an area . . 0. danae (Meek and Worthen), p. 356. 



*Oligoporus blairi Miller and Gurley. 

 Plate 47, figs. G, 7; Plate 48, fig. 1; Plate 49, figs. 1-3. 



Oligoporm blairi Miller and Gurley, 1893, p. 6, Plate l.figs. 2,6; Plate 2, fig. 7; Lambert and Thiery, 

 1910, p. 121. 



Oligoporus bellulus Miller and Gurley, 1893, p. 7, Plate 1, fig. 3; Umbert and Thiery, 1910, p. 121. 

 Oligoporus danae (pars) Klem, 1904, p. 37. 



The test is spheroidal, with moderately developed melon-like ribs. In one specimen 

 (Plate 49, fig. 1) the ambulacra measure 12 mm., the interambulacra 21 mm. in width. In a 

 second specimen (Plate 49, fig. 3) the ambulacra measure 12 mm. and the interambulacra 

 18 mm. in width. 



The ambulacra are relatively wide, having at the mid-zone four columns of narrow derni- 

 and wider occluded plates, also some scattered isolated plates in or near the middle of the half- 

 areas. Occluded plates are elevated in a low rounded median ridge; pore-pairs are near the 

 outer border of each ambulacral plate. 



1 The species described as Oligoporus (?) minutus is considered under Incertae Sedis (p. 450). 



