26 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
Among the questions discussed by us in Part II., the 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 
Palzeocrinoidea ; but the difficulty was 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 Paleocrinoidea 
were subdivided into three sub-orders: the CAmERATA, the INADUNATA, 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 paleontological record, they 
being already highly differentiated among the earliest known Crinoids. 
We divided them into twenty-six families. 
The Crinoidea Camerata were separated into Reteocrinide, Rhodocrinide, 
Thysanocrinide, Glyptasteride, Melocrinide, Actinocrinidx, Platycrinide, 
Hexacrinidx, Eucalyptocrinide, Barrandeocrinidx, and Acrocrinide. 
The Crinoidea Articulata were subdivided into Ichthyocrinide and Cro- 
talocrinide. 
The Crinoidea Inadunata were subdivided into ;— A. Crinoidea Larviformia, 
with Haplocrinidw, Cupressocrinide, Gasterocomide, and Stephanocrinide ; 
B. Crinoidea Fistulata, with Hybocrinide, Heterocrinide, Anomalocrinide, 
Belemnocrinida, Cyathocrinide, Calceocrinide, Catillocrinide, Poteriocri- 
nidw, Encrinidx, and Astylocrinide. 
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- 
