24 THE CRINOIDEA CAMEEATA OF NOETH AMEEICA. 



boscis with the opening at the upper end : Gasterocomidce^ Platyainidce, 

 Carpocrinidce^ Briarocrinidce, Dimerocrinidce, Barrandeocrinidce^ AcUnocrinidcB, 

 Stelidiocrinidce, MelocrinidcB^ PolypeltidcB, Uintacrinidce, Glt/ptocrinidce and Rho- 

 docrinidce. 



E. Tegmen bottle-shaped, narrower at the top ; composed of large, 

 polygonal plates, regularly arranged. Arms not extended beyond the calyx, 

 biserial ; placed between rib-like projections, or occupying closed compart- 

 ments : Gali/ptocrinidce. 



The Articidata were defined by Zittel as follows : Plates of the dorsal cup 

 generally very heavy, the articular faces excavated or smooth. Base mono- 

 cyclic, and formed of five pieces -, exceptionally dicyclic. Tegmen, as a 

 rule, composed of a skin -like perisome, rarely plated ; mouth and food 

 grooves exposed. The mouth central ; anus excentric ; orals present or 

 absent. Plates of the dorsal cup perforated by axial canals, passing out 

 from the dorsal organ, and continued along the solid parts of the arms to 

 the ends of the pinnules. 



To the Articidata he refers the following families : the EncrinidcB, Eiigenia- 

 crinidce, Holopidoe, Plicatocrinidce, Apiocrimdce, Pentacrinidce and ComatiilidcB. 



Examining the characters upon which Zittel separated the Tessellata 

 from the Articulata, it appears that not one of his distinctions holds good 

 throughout the families of either group. The '^ Taxocrinidge " and " Ichthyo- 

 crinidaB," which he refers to the Tessellata, have a more flexible disk than 

 either the Encrinidse or Apiocrinidae, and even than many of the Penta- 

 crinidge ; and, besides, have an open mouth and open food grooves. The 

 Poteriocrinidge have a flexible disk, and w^ell defined muscular articulations 

 between the radials and costals. The interbrachials of Giiettardiciiniis and 

 Apiocriims roissyamis are as heavy and rigid as those of any Actinocrinus, and 

 the plates of the dorsal cup in Eiipachi/crinus and Prisocrimis are perforated 

 by axial canals, a character which was supposed to occur only among the 

 Articulata. Neither is it true that the Articulata are ^^ rarely dicyclic " ; 

 the reverse would be more nearly correct, for most of them had small infra- 

 basals in early life, which gradually fused with the stem. No doubt Zittel's 

 groups marked A and E form excellent divisions, but B, C, and D comprise 

 widely differing types, and some of their families include monocyclic and 

 dicyclic forms. As a whole, Zittel's classification marks a great advance 

 over those of his predecessors, and he is the first writer who gave a good 

 definition of the. families, and who arranged them systematically. 



