ACEOCEINID.E. 809 



The basals are followed by from fifteen to twenty rings of accessory pieces — 

 the number varying among the specimens — and each ring consists of twenty 

 to twenty-four plates, except the two upper rings, which contain but twelve. 

 Of the upper ring, seven plates are arranged interradially — three occupy 

 the anal side and one the regular sides — which alternate with the radial 

 pieces. Most of the accessory pieces are hexagonal, and angular at top 

 and bottom ; but at the anterior and posterior sides those constituting the 

 middle series are truncated at both ends. Radials large, one third wider 

 than long, deeply excavated at the upper face into a facet which contains 

 the costal and both distichals. Anal plate of the same width as the radials 

 but considerably longer. It supports several small plates, which project 

 outward, and may be readily taken for the base of an arm. Costals wider 

 than in the two preceding sjjecies, but equally short; the distichals propor- 

 tionally longer and narrower, rounded on the back, and distinctly curving 

 outward. Palmars free from the second or third plate, curving outward and 

 downward, bringing the arms into a pendent position, their backs next to the 

 calyx, and the ambulacra and pinnules on the outside. Arms twenty, moder- 

 ately long, descending far beyond the basals ; they are composed from the 

 fourth or fifth f>late of two series of rather short joints, and rest, apparently 

 immovably, with their backs within deep longitudinal grooves or impressions 

 on the surface of the dorsal cup. Pinnules long, closely jjacked ; composed 

 of long joints three times as long as wide, their ventral furrows, like those 

 of the arms, lined by two rows of covering pieces. Ventral disk flat, 

 tlie middle portions composed of numerous small perisomic plates, from 

 which well defined ambulacra! pieces, alternately arranged and suturally 

 united, curve downward, and pass out into the arms. Anus eccentric, on 

 a level with the upper surface, placed about half way between the centre of 

 the disk and its outer margin. Column round, the joints nearly of equal 

 size. 



Horizon and Locality. — St. Loins group ; near Huntsville, Ala., where 

 we obtained a considerable number of specimens. 



Tj-pes in the collection of Wachsmuth and Springer. 



Remarks. — This species is readily distinguished from A. Sliimiardi by 

 its smaller size and recumbent arms, and from A. Woriheni by the different 

 form of the calyx and the much smaller number of accessory pieces of that 

 species. 



The arms of this species were apparently immovable, as is indicated by 



102 



