62 BULLETIN 64, UNITED STATES NATIONAL MUSEUM. 



drawn out to occupy the space between the wide primaxil and the 

 adjacent interbrachial. Primaxils three times as wide as long. 

 Secundibrachs but slightly smaller than the primaxils. In the antero- 

 lateral rays, which have three arms, the secundibrachs are followed 

 on the posterior side by two series of tertibrachs, and on the anterior 

 side by two additional secundibrachs, which support the arms. The 

 posterior rays are symmetrical, having on each side one secundibrach 

 followed by 2 x 2 tertibrachs. The plates of the anterior ray are not 

 preserved beyond the first secundibrachs, but the arm formula for the 

 species, so far as it can be determined, is 1-2 . . . 2-2 . . 



A . . 2-2 . . . 2-1. 



First interbrachials long, reaching from the radials to the top of 

 the arm bases, and succeeded by two small plates, which form a part 

 of the tegmen. Anal area of moderate width. Anal plate longer 

 than the radials, succeeded by a plate of about half its size and by 

 two large plates, which rest against its superior lateral slopes. These 

 plates are followed by numerous small tegmen plates of indefinite 

 arrangement. 



The plates of the dorsal cup are convex but not nodose. The con- 

 vexity renders the suture lines between the plates very distinct. The 

 outer surface is not preserved. 



The height of the tegmen is about equal to its greatest diameter. 

 Orals in contact, large and moderately convex. Lines of nodose 

 plates cover the ambulacral areas from the orals to the arm bases, 

 terminating in two large plates on the four-armed rays, and in a large 

 and small plate on the three-armed rays. A single large plate over 

 the anterior ray indicates that it possessed only two arms. Interam- 

 bulacral plates slightly or not at all convex. Anus situated about 

 halfway between the base and the apex of the tegmen. Aperture 

 not elevated above the general surface of the tegmen, but the plates 

 just above the anus are raised into a slight swelling of the surface. 



Observations. — This species has the projecting arm bases attributed 

 to Agaricocrinus crassus Wetherby, but the whole structure of the 

 crinoid is characterized by lightness and delicacy in contrast with 

 the massive structure of A. crassus. The calyx plates above the 

 radials are very short and wide, unlike those of A. crassus, and A. 

 attenuata has no anal protuberance. 



A. elegans Wetherby agrees with A. attenuata in being less robust 

 than A. crassus, but it differs in the number of arms, having three or 

 four in the posterior rays and two in all the other rays. It also differs 

 in having short first interbrachials and an extremely wide anal area. 



Formation and locality. — Keokuk horizon of the Tullahoma forma- 

 tion. White's Creek Springs, Tennessee. 



Cat. No. 39976, U.S.N.M. 



