6 



Ambulacral areas as wide in the middle part, and wider 

 toward the poles than the interambulacral areas, and separated 

 in each ambulacral field the entire length by a sharply defined 

 ridge elevated as high as the interambulacral areas. The plates 

 are smaller and more irregular in the ambulacral depressions 

 than elsewhere. The ambulacral pores are circular, two in each 

 plate, and form four irregular ranges of pairs in each depres- 

 sion. When the two depressions, in each ambulacral area, unite 

 near the ocular plate there are five or six rows of pores. The 

 small plates are pierced with the ambulacral pores until they 

 actually abut upon the little ocular plates. 



Three pores are clearly distinguishable, in some of the genital 

 plates, with an ordinary magnifier, and there are some evidences 

 of other pores that have been closed or filled up. There are no 

 pores in the ocular plates. The plates abutting the oral open- 

 ing are angular and preserve the articulating edges of the oral 

 plates. 



Found in the St. Louis Group, at Greenville, Harrison county, 

 Indiana, and now in the collection of Wm. F. E. Gurley. 



OLIGOPORUS BLAIRI, n. Sp. 



Plate I, Fig. 2, the upper part of a specimen, somewhat crushed, 

 and preserving none of the apical plates; Fig. 6, a 

 smaller specimen, crushed so as to show only part 

 oi the ambulacral pores; Plate II, Fig. 7, 

 the middle part of a much larger 

 and flattened specimen. 



The three specimens enable us to ascertain most of the exter- 

 nal characters of this species, excepting the apical and basal 

 plates, none of which are preserved. If they all belong to the 

 same species, as we think, then they show great differences in 

 the size of the plates and in the number longitudinally, without 

 increase in the number of ranges, in either the ambulacral or 

 interambulacral areas. In other words, if the three specimens 

 belong to the same species, the plates increase, in size, with the 

 growth, by accretion laterally, without intercalation of plates, 

 but increase longitudinally by the intercalation of plates, and 

 very little by accretion. We would not be understood as affirm- 



