OLIGOPORUS. 355 



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

 occluded plates are medially rounded up in a high, steeply arcuate ridge. Pore-pairs arc in 

 deep, sunken valleys, lying in the outer border of each ambulacral plate. 



The interambulacra are wide, strongly elevated in melon-like ribs, which outwardly present 



* 



a gently rounded curve, but laterally dip suddenly down to the adradial sutures. At the mid- 

 zone in each area there are six columns of interarnbulacral plates, but just above the mid-zone, 

 a seventh column appears in each area. As noted by Miller and Gurley, part of the columns 

 drop out dorsally before reaching the apical disc, indicating senescence. Traces of insert ocular 

 and genital plates appear dorsally. This species is close to blairi and coreyi, but differs from 

 them, as discussed under coreyi. Its reference to Melonechinus multiporus as a synonym by 

 Miss Klem is certainly wide of the mark. 



St. Louis Group, Lower Carboniferous, Hardin County, Kentucky, holotype in University 

 of Chicago Collection 6,623, whence it was kindly loaned me by Professor Stuart Weller. 



*01igoporus halli sp. nov. 

 Plate 47, figs. 11, 12; Plate 48, fig. 5. 

 Oligoporus sp. Klem, 1904, Plate 1, figs. 3a, 3b. 



A large, massive sea-urchin, slightly elliptical in form, with moderately developed melon- 

 like ribs. Height of type 103 mm., diameter through the mid-zone 100 mm., width of ambu- 

 lacra at the mid-zone 18 mm., width of interambulacra 31 mm. 



Ambulacra near the mid-zone with four columns of demi- and occluded plates, the latter 

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

 cluded plates in the middle are curved outward in a low, rounded arch. Pore-pairs in lateral 

 depressions, are situated in the outer portions of each ambulacral plate. 



Interambulacra are wide, rounded up from the adradial sutures in a gentle rounded curve. 

 In areas A, C, and I there are eight columns of interambulacral plates at or just above the mid- 

 zone, but in areas E and G there are only seven columns. Areas G and I are not seen in the 

 photographic figure (Plate 48, fig. 5), being on the opposite side of the specimen. In area A 

 (Plate 47, fig. 11) the two ventral rows of plates are restored, as indicated by dotted lines; there 

 are four plates in the third row. The fifth column appears in the sixth row and the sixth column 

 in the eleventh row. The seventh column appears on the left of the center in the fifteenth row. 

 In area C the seventh column appears in the center five rows above the base as far as pre- 

 served, and the eighth column originates at or just above the ambitus. The introduction of 

 these two columns in area C is clearly seen in the photographic figure (Plate 48, fig. 5). The 

 apical disc and dorsal portion of the specimen are wanting. 



This fine species is known only from the holotype which was kindly loaned me by Mr. 



