9 



The viiult is exceedingly roiivcx aiul very slightly ileiJreHsed in the 

 interradial areas. There is a large tuuiitl plate at the apex of the 

 vanlt. which is surroLinded 1)V seven large tumid plates. There are 

 three large tumid i)lates over the junetion of the ambulacral canals 

 in each of four of the radial series, and one over the junction of the 

 ambulacral canals, in the ray opposite the azygous area. The other 

 plates of the vault an' mucli smaller, but very irregular in size, the 

 smallest ones are in the regular interradial and azygous areas. The 

 azygons area is wklc and covered by numerous polygonal plates. The 

 azygous orifice is at tlie suj);M'ior ])art of a l)ulbous swelling and sur- 

 rounded by small jilates. 



This is tlie first sixteen-armed species ever described except the 

 Arl/i/orriinis ( .^) hclicc of Hall to which it. bears very little, if any, 

 resemblance. It would seom to be as nearly related to .1. ioireiisi^, 

 above described, as to any other spi'cies. but it is widely different 

 from that species, in the azygons and interradial areas, in the sec- 

 ondary and tertiary radials, and in the plates of the vault, beside 

 having sixteen arms while that species has only fifteen. It is a very 

 strongly marked species. 



Found in the Keokuk (rroup. at Keokuk. Iowa, and now in the 

 collcciioii iif \Vm. I'"'. E. (iiirley. 



EeM.AKKS ON" A(i.\Hia)('RINUS. 



A(/(iric(iri'lini>: is a well marked genus that does not graduate into 

 any other nor toward any other through any of the species belong- 

 ing to it. It is only known from the lower half of the Subcar- 

 boniferous System. It first makes its appearance in the Chouteau 

 limestone. Here we find A. hlairi, which has a subquadrate flattened 

 body and liears oidy nine arms. The ray opi)osite the azygous area 

 bears a single arm, and each of the other rays bears two. It is the 

 only nine-armed species tluit has been des<!ribed, and has very little 

 resemhiance in either form or structure to any genus or species that 

 preceded it. There have been described, also, from the Chouteau 

 limestone, three ten-»rmed species, viz.: ^-1. fieriiid nils, which is a 

 small species, having a ilattened body and a tumid jilate above each 

 ambulacral orifice; the vault is covered by fewer plates than any 

 other known species; A. rlioiitcfnieiisifi, which has an abrupt basal 

 depression, subconical vault, and unequal radial series: and ^4. 

 >i((mj)Soni, which bears very large arms, in proportion to the size of 

 the flattened calyx. These species are widely separated from each 



