7 



ing positively that this is the law governing the growth of this 

 species, for if so, it would, probably, be the law in the growth 

 of all species in this genus, and also in the family. We have 

 not enough evidence in the three specimens before us, injured as 

 they are, to positively affirm that they belong to the same 

 species, but we think the weight of the evidence is in that direc- 

 tion, and hence we suggest the probability of the method of 

 growth, the correctness of which will be determined by the ex- 

 amination of other specimens. 



The interambulacral areas are lance-elliptical, in outline, mod- 

 erately and evenly convex from one ambulacral depression to 

 another. There are six ranges of plates at the middle part. The 

 plates where well preserved, are covered with tubercles that ap- 

 pear to have been the bases of spines, though none of the spines 

 are preserved on our specimens. 



The ambulacral areas are about half as wide, in the middle 

 part, as the interambulacral areas, and of almost uniform width 

 throughout their length, tapering very slowly as they approach 

 the summit and ocular plates. They are separated in each am- 

 bulacral field, by a sharply defined ridge elevated higher than 

 the interambulacral areas. The plates are short and small in 

 the ambulacral depressions and each one is pierced with a pair 

 of circular pores. There are two ranges of these ambulacra! 

 pores in each depression throughout the length without the 

 addition or intercalation of any plates or pores. 



This species is distinguished from 0. danw, by the fact, that 

 there are only six instead of eight ranges of plates in each in- 

 terambulacral area. 



Found in the Keokuk Group, at Boonville, Missouri, and now 

 in the collection of both authors. 



OLIGOPORUS BELLULUS, n. Sp. 



Plate I, Fig. 3, part of the upper half of a crushed specimen. 



The surface of the plates, in our specimen, is covered with 

 tubercles, that formed the bases of spines, and some of the 

 spines are scattered over parts of the ambulacral depressions. 

 The spines are minute, elongated, tapering, cylindrical in section, 

 swollen a little at the basar extremity, and about one-tenth of 



