pose “A —— Se EEN Se * kee; Se Le Se RASS Sie ne eS aN 4 % s e ’ rae = : . . 
os / Bass Pe Oe ; JE fap SS ee ag ie ~, ~ eS (SSE NSS te Ses aes Gree ae . % 
=e 2 AE rn EE re _ Pet ho a 
ACROCRINID&. 805 
than probable that Acrocrinus represents phylogenetically a highly differen- 
tiated form of Dichocrinus. This is further confirmed by the fact that 
g Acrocrinus, while in part contemporaneous with Dichocrinus, 1s, so far as 
known, the last surviving genus of the Camerata. | 
ACROCRINUS YANDELL. 
1855. Yanperi; Amer. Journ. Sci. and Arts, Vol. XX. (new ser.), p. 135. 
1858. Hatt; Geol. Rep. Iowa, Vol. I., Part II., p. 589. 
1882. WacusmutH; Bull. I. Illinois State Museum, p. 41; and Geol. Rep. Illinois, Vol. VIIL., p. 348, 
Plate 30, Fig. 13. 
1885. W.and Se.; Revision Paleocr., Part III, p. 124 (Proceed. Acad. Nat. Sci. Phila., 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. Radials 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. Radials 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 Acrocrinide, 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. 
Tyne of the genus: Acrocrinus Shumardz. 
Remarks. — 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 conelusively 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. Worthen, 
in which the lower face of the radials rests upon a single plate, which natu- 
