26 THE CEIISrOIDEA CAMEEATA OF NOETH AMEEICA. 



Among the questions discussed by ns in Part IL, tlie most perplexing 

 one was that respecting the orals. As these plates are developed among the 

 first in the larva, it seemed to follow that they must be present also in the 

 Palaeocrinoidea ; but the difficulty w^as to determine which of the various 

 plates in the summit represent them. The median portion of the tegmen 

 in most of those Crinoids is covered by from seven to ten asymmetrically 

 arranged, more or less prominent plates, consisting of a central one of larger 

 size, four others nearly as large directed anteriorly, and five others placed 

 posteriorly. We regarded the central plate as an element unknown among 

 the later Crinoids, which occurred only in forms with a closed mouth, and 

 that it actually covered the oral opening. The orals, we thought, were 

 represented by six plates : four large and two smaller ones, the two latter 

 jointly representing the posterior oral, broken up on account of anal plates. 



Part III. of our Revision appeared in 1885. It contains the genera which 

 had not been considered before ; and we also completely revised Parts I. and 

 II., making important changes in the classification. The Palasocrinoidea 

 were subdivided into three sub-orders : the Camerata, the In"ADUKATA, and 

 the Articulata, which we distinguished by the mode of union between the 

 plates of the calyx, and the condition of the arms, whether free above the 

 radials, or partly incorporated into the cup. The origins of these groups 

 date back to a time of which we have no palseontological record, they 

 being already highly differentiated among the earliest known Crinoids. 

 We divided them into twenty-six families. 



The Crinoidea Camerata were separated into Reteocrinidse, Ehodocrinidge, 

 Thysanocrinidas, Glyptasteridae, Melocrinidae, Actinocrinidge, Platycrinidse, 

 Hexacrinid^, Eucalyptocrinidse, Barrandeocrinidaa, and Acrocrinidse. 



The Crinoidea Articulata were subdivided into Ichthyocrinidse and Cro- 

 talocrinidse. 



The Crinoidea Inadunata were subdivided into ; — A. Crinoidea Larviformia, 

 with Haplocrinidae, CupressocrinidaB, Gasterocomidse, and StephanocrinidaB ; 

 B. Crinoidea Fistulata, with Hybocrinidse, Heterocrinidge, Anomalocrinidae, 

 Belemnocrinid^, Cyathocrinidse, Calceocrinidae, Catillocrinidse, Poteriocri- 

 nidae, Encrinidse, and Astylocrinidae. 



Most important from a morphological point of view was the discovery 

 of a fixed law respecting the orientation of the stem, which enabled us to 

 ascertain the presence of infrabasals in many species in which these plates 

 are hidden by the column. By means of this law we were led to the con- 



