ACROCRINID^. 805 



than probable that Acrocrinus represents phylogenetically a highly differen- 

 tiated form of Diclioainus. This is farther confirmed by the fact that 

 Acrocrinus, while in part contemporaneous with Dichocrinus, is, so far as 

 known, the last surviving genus of the Camerata. 



ACROCRINUS Yandell. 



1S55. Yandell; Amer. Joum. Sci. and Arts, Tol. XX. (new ser.), p. 135. 



1858. Hall ; Geol. Rep. Iowa, Vol. I., Part II., p. 689. 



1882. Wachsmuth; Bull. I. Illiuois State Museum, p. 41; and Geol. Rep. Illinois, Tol. VII., p. 313, 



Hate 30, Kg. 13. 

 1S85. W. and Sp. ; Revision Palaeocr,, Part III., p. 134 (Proceed. Acad. Nat. Sci. Pliila., p. 346). 



Calyx amphora- or urn-shaped, longer than wide. Basals two, equal, 

 forming a low basin or flat disk ; the suture line passing from the anterior "to 

 the posterior side ; the upper face straight. Eadials separated from the 

 basals by an indefinite number of supplementary pieces placed in rows, alter- 

 nately arranged, except those supporting the anal plate and anterior radial ; 

 those of every succeeding row are somewhat larger. The plates are in part 

 radial and in part interradial, the latter considerably the most numerous at 

 the posterior side; the uppermost row supports the radials, which are moder- 

 ately large and enclose an anal plate. Eadials at their upper faces broadly 

 and deeply excavated for the reception of the costals and distichals. Costals 

 very small, subtrigonal. Distichals two, the lower one placed against the 

 sloping faces of the costals and upon the radials. Palmars two if there is 

 another bifurcation. Arms biserial, either erect or pendent ; in the latter 

 case the ambulacra exposed. Pinnules long and closely packed. Ventral 

 disk flat, composed of small plates. Anal opening eccentric, near the margin 

 of the disk. 



Distribution. — Acrocrinus, the only genus of the Acrocrinidse, is repre- 

 sented, so far as known, only in America, and there only by three species, 

 which range from the St. Louis group to the Coal Measures. 



Ti/pe of the genus : Acrocrinus Sliumardi. 



Remarhs. — In the Revision, Part III., we described this genus as having 

 " three radials," or, as we would say now, a radial and two costals. Since 

 then we have obtained very much better specimens than were ever known 

 before, and these prove conclusively that the so-called first radial is merely 

 an accessory piece, and that the plate above it is the true radial. Our 

 interpretation seemed to be quite reasonable in the case of A. Wortheni, 

 in which the lower face of the radials rests upon a single plate, which natu- 



