520 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
distichals. Arms branching in the biserial state; long, slender, rounded ; 
constructed from their bases up of a double series of interlocking pieces. 
Pinnules slender, closely contiguous. Interbrachial and interdistichal spaces 
long and narrow, composed of a large number of plates. First interbrachial 
= 
ee 
larger than the succeeding ones; supporting numerous rows of two plates each 
— rarely three, and only in the upper rows — which pass imperceptibly into 
interambulacral pieces. Anal interradius very wide; the first anal similar 
in form to the radials; there are three plates in the next range, and four to 
six In the succeeding ones. Interdistichals present, and variously repre- 
sented by from three to six rows. Ventral disk depressed, from moderately 
convex to almost flat; composed entirely of small, irregularly arranged plates, 
apparently without orals or radial dome plates, except close to the arms, where 
sometimes the covering and side pieces enter the margin. Anus subcentral. 
Column large, cylindrical ; axial canal moderately wide and circular. 
Distribution. — Periechocrinus is found in the Niagara group of America, 
and its equivalent in England and Sweden. It also occurs slightly modified 
in the Upper Devonian of Spain, and in the Lower Subcarboniferous of the 
Mississippi Valley. 
Type of the genus: Periechocrinus moniliformis (J. S. Miller). 
Remarks. — Austin’s definition of Periechocrinus is not as clear as could be 
wished, but it is enough to indicate that he refers to a group of Crinoids of 
which J. 8. Miller’s well known Actinocrinus moniliformis is the type. The 7 
species are remarkable for their elongate, sac-like form, the thinness of the 
plates, the great length and slender form of the radials, the width of the anal 
interradius, and the branching of the biserial arms, substantially the same 
characters upon which afterwards Hall undertook to separate the genus 
Saccocrinus. 
In the Revision, Part II., we placed under Periechocrinus not only the 
ie 9 ee a lara el me? ne en 
species which had been referred to Saccoerinus, but several others from the 
Lower Carboniferous which had been described variously under Actinocrinus, 
Megistocrinus, Saccoerinus, and Pradocrinus. These species, which embrace 
{ , Actinocrinus (Megistocrinus) Whitet Hall, Actinocrinus (Pradocrinus) amplus 
aie i M. and W., and Actinocrinus tenuidiscus Hall, resemble Pertechocrinus in 
general habitus, but come from a very different geological horizon, have 
a less elongate calyx, and proportionally shorter plates. They differ still 
K | more from Megistocrinus by the thinness of their plates, and by having 
Hy radial ridges. ‘These species we have marked with a query, as it Is pos- 
eo 
ee ee 
aa ~ 
in I I FR IRR eer secon I nt re 
Sa Se 
— " oe — Sacco TR TTT 
