170 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



CAMERATA. 



Analysis of the Families, 



I. Typical Section. 



Lower brachials and interbrachials forming an important part of the 

 dorsal cup. 



A. Interradials ill defined. 



The lower plates of the rays more or less completely separated 

 from those of other rays, and from the primary interradials, 

 by irregular supplementary pieces. Anal interradius divided 

 by a row of conspicuous plates. (Dicyclic or Monocyclic) . Reteocrinid^. 



B. Interradials well defined. 



1. Dicyclic. 



a. Radials in contact exceiot at the posterior side Thysanocrinid^. 



b. Radials separated all around Rhodocrinid^. 



2. Monocyclic. 



a. Radials in contact all aroimd. 



Symmetry of the dorsal cup, if not strictly pentamerous, 

 disturbed by the introduction of anals between the 

 brachials only Melocrinid^. 



Arms borne in compartments formed by partitions attached 

 to the tegmen. Dorsal cup perfectly pentamerous. Plates 

 of calyx limited to a definite number Calyptocrinid^. 



b. Radials in contact except at the posterior side, where they 



are separated by an amal plaM. 

 Eirst anal plate heptagonal, followed by a second between 



two interbrachials Batocrtnid^. 



Eirst anal plate hexagonal, followed by two interbrachials 



without a second anal. Arms branching from two main 



trunks by alternate bifurcation Actinocrinid^. 



II. Non-Typical Section. 

 Brachials and interbrachials but slightly represented in the dorsal cup. 



1. Monocyclic. 



a. Radials in contact all around. Base pentagonal .... Platycrinid.35. 

 h. Radials separated at posterior side by an anal plate. Base 

 hexagonal. 



Basals directly followed by the radials Hexacrinid^. 



Basals separated from radials by accessory pieces . . Acrocrinid.e. 



2. Dicyclic. 



Radials in contact except at the posterior side Crotalocrinid^. 



