520 THE CEINOIDEA CAMEEATA OF NORTH AMEEICA. 



disticbals. Arms branching in the biserial state ; long, slender, rounded ; 

 constructed from their bases up of a double series of interlocking ijieces. 

 Pinnules slender, closely contiguous. Interbrachial and interdistichal spaces 

 lono- and narrow, composed of a large number of plates. First interbrachial 

 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. 



Disiribidion. — Pericchocrinus 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 : Periechocriniis moniliformis (J. S. Miller). 



Bemarhs. — Austin's definition of Periechocriniis 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. S. Miller's well known Actinocrinus moniUformis is the type. The 

 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 Periechocrimis not only the 

 species which had been referred to Saccocrinus, but several others from the 

 Lower Carboniferous which had been described variously under Actinocrinus, 

 Megisiocrinus, Saccocrinus, and Pradocrimis. These species, which embrace 

 Actinocrinus {Megistocrinus) Whitei Hall, Actinocrinus {Pradocrimis) amjulus 

 M. and W., and Actinocrinus tenuidiscus Hall, resemble Perieclwcriniis in 

 general habitus, but come from a very different geological horizon, have 

 a less elongate calyx, and proportionally shorter plates. They differ still 

 more from Megistocrinus by the thinness of their plates, and by having 

 radial ridges. These species we have marked with a query, as it is pos- 



