OLIGOPORUS. 353 



right ventral border. Three rows higher again appears the initial plate of the seventh column 

 with a heptagon on its left ventral border. 



This species is very close to Oligoporus coreyi; but apparently differs from that species 

 in the less pronounced and less sharply elevated ribs in the arnbulacral areas. It is very distinct 

 from Oligoporus danae of which it was treated as a synonym by Miss Klem. Through the 

 kindness of Professor Stuart Weller, I have been able to study all of the original material and 

 give figures of the same. 



Keokuk Group, Lower Carboniferous, Boonville, Missouri, cotypes, three specimens in 

 the University of Chicago Collection 6,470; same locality, an additional specimen (which is the 

 holotype of Oligoporus bellulus) University of Chicago Collection 6,190; Keokuk Group, Bono, 

 Lawrence County, Indiana, Museum of Comparative Zoology Collection 3,188. 



*Oligoporus coreyi Meek and Worthen. 

 Plate 47, figs. 8, 9; Plate 48, fig. 2. 



Oligoporus coreyi Meek and Worthen, 1870, p. 34; Love"n, 1874, p. 42; Keyes, 1895, p. 183; Jackson, 

 1896, p. 189, text-fig. 1, p. 191, Plate 6, figs. 25, 28, 29; Klem, 1904, p. 38; Lambert and ThieVy, 

 1910, p. 121. 



The type specimen is silicified and beautifully preserved as regards form, although not very 

 clear in detailed structure, as it is worn, and the test is somewhat compressed. Test spheroidal, 

 with strong, rounded, elevated melon-like ribs in both ambulacral and interambulacral areas. 

 Height 40 mm. ; this measurement would be greater if the specimen had not been compressed. 

 Diameter at the mid-zone through the plane B, G, 58 mm.; through the plane D, I, 59 mm. 

 The measurements of the diameter are somewhat exaggerated by the dorso-ventral compression 

 of the test. The ambulacra at the mid-zone measure about 12 mm. in width and the inter- 

 ambulacra about 20 mm. in width. 



The ambulacra could not be made out satisfactorily at the mid-zone, but in the mid-dor- 

 sal region (Plate 47, fig. 9) there are four columns of demi- and occluded plates. These are of 

 about equal width, a usual character dorsally; but at the mid-zone the occluded are probably 

 wider than the demi-plates. In addition, there are some scattered isolated plates that do not 

 reach either the middle of the area or the interambulacra, which is the generic character. 

 Pore-pairs are in deep sunken furrows, lying in the outer border of each plate. The occluded 

 plates in the middle are arched up in a high rounded melon-like curve. 



The interambulacra just above the mid-zone have seven columns of plates in areas A, E, 

 and G. In area C there are only six columns, and in I there are six columns, and possibly 

 seven, but this area is not clear dorsally. The seventh column comes in a little dorsal to the 

 mid-zone, and at that zone, strictly speaking, there are six columns of plates in all areas. The 



