16 



the last being axillary and supporting, upon each outer sloping 

 side, a free arm, which gives to each ra}' four arms. There are, 

 therefore, twenty arms in this species. 



In each regular interadial area there are five plates; one, followed 

 by two in the second range, and two in the third range, which 

 widely separate the arms and unite with the plates of the vault. 

 In one area, however, the first plate is divided horizontally, so as 

 to give the area six plates, which may or may not be abnormal. 

 The azygous area is wide and the plates of the calyx graduate 

 into those of the vault, without any distinct line of separation. 

 The first plate is in line with the first primary radials and fully 

 as large as either of them. It is followed by three plates, in the 

 second range, the middle one being the smallest. There are four 

 plates in the third range and four in the fourth range, the latter 

 being of unequal size and forming an irregular range. Some of 

 the plates, in the fifth range, abut upon the plates that sur- 

 round the azygous orifice, which is situated upon the side, below 

 the summit of the vault. 



The vault is highly convex and has greater capacity than the 

 calyx itself. There is a very large subcentral plate surrounded by 

 eight plates, four of which are large, two are medium size, and 

 the other two are smaller and abut upon the azygous orifice. The 

 cast shows a pit in each of the four large plates, and in the in- 

 terambulacral plates below, but there are none, in the ambulacral 

 plates. There is also a pit subcentrally where the large subcen- 

 tral plate rested. Probably these pits indicate spine-bearing 

 plates. 



This species bears little or no resemblance to any twenty-armed 

 species heretofore described, and the test, if ever discovered, can 

 probable be identified. At least, the test is as likely to be iden- 

 tified from the cast as the cast would have been from the test, if 

 we had described the test instead of the cast. Twenty-armed 

 species prevail, in the Keokuk Group, over other forms, 

 and twelve and sixteen-armed species prevail in the Burlington 

 Group, over other forms. The general form of this species is 

 more like the prevailing forms in the Keokuk, than the prevailing 

 forms in the Burlington, and without having examined the rocks 

 from which it was collected, we suppose it was from the Keokuk 

 Group. 



