TKOOST'S CRINOIDS OF TENNESSEE E. WOOD. 



61 



the primaxil is followed by three or four secundibrachs, of which the 

 distal plate gives rise to an arm. This gives the following arm 

 formula for the species : a 1-1 . . . 1-2 . . . 2^-2 . . A . . 2-2 . . . 

 2-1 . The arms are not preserved. 



The first interbrachials are long, reaching from the superior angles 

 of the radials to the top of the arm bases. Unlike most species of 

 Agaricocrinus , the first interbrachials are followed by three plates, 

 which form a part of the tegmen. A single plate truncates the distal 

 end of the interbrachial, and two large plates abut against its superior 

 lateral slopes. The anal plate is concealed by the matrix. It is 

 followed by three large plates, which curve over the margin of the 

 calyx and unite with plates of the tegmen. 



Tegmen of heavy, nodose plates. Orals in contact. Posterior oral 

 nearly twice as large as the others and more convex. Interambu- 

 lacral plates comparatively small and but slightly convex. Anal 

 aperture about one-third the distance from the apex to the base of 

 the tegmen, and situated at the center of an oval, moderately elevated 

 prominence. 



The surface of the plates within the basal excavation appears 

 smooth, but on the arm bases and the tegmen it is rather coarsely 

 granulose. 



Observations. — This species is most closely related to A. nodulosus 

 Worthen, but is distinguished by the absence of nodose calyx plates 

 and by the presence of very long first interbrachials, which are fol- 

 lowed by one instead of two plates. A. ~keokukensis Miller and Gur- 

 ley, which has the same number of arms, has very small primaxils 

 and strongly nodose secundibrachs, and the interbrachial areas are 

 occupied by three small plates following one another in radial series. 



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

 tion. White's Creek Springs, Tennessee. 



Cat. No. 39975, U.S.N.M. 



AGARICOCRINUS ATTENUATA, new species. 

 Plate 14, figs. 3, 4. 



Agaricocrinus tuberosus Troost (in part), Proc. Amer. Ass. Adv. Sci., II (read 

 1849), 1850, p. 60 (nomen nudum). 



This species is characterized by a body of moderate size, with 

 slender, projecting arm bases. 



Basals nearly concealed by the matrix but apparently projecting 

 a little beyond the column. Radials about equal in length and 

 width. First primibrachs irregularly quadrangular, one corner being 



«The arm formula used in this paper is the one proposed by the writer in the Smith- 

 sonian Misc. Coll., XLVII, 1894, p. 69, with the interpolation of the letter A in the 

 position of the anal area for forms in which that structure is well differentiated. 



