62 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
stem joint of Millericrinus Orbignyi and M. polyjdactylus ; while in other species 
of the Apiocrinide in which infrabasals are unrepresented, the column 
touches the basals only. It rests against the outer (dorsal) surface of the 
plates, whose ower margins are bent up, so as to form an inverted pentan- 
gular concavity, radially disposed. This cavity, which is occupied and 
completely filled by the top stem joint, is generally grooved at the inter- 
basal sutures, and produces upon the upper face of the joint five angles, 
which fit into the grooves, and interlock with the basals (Plate VI. Figs. la, 
14, and 5), The outer edge of the joint at the upper end has also a more or 
less radial outline; while its lower end follows the orientation of the stem, 
and is interradially disposed when that is pentangular. 
We mention these particulars, because Carpenter, in criticising our gen- 
eralization,* undertook to prove by the angularities at the upper face of the 
stem that in those species of the Apiocrinid in which the stem is round, 
the latter was radially disposed, and not interradially; and that those 
species, according to our own rules, were monocyclic and not dicyclic. He 
overlooked the fact that the top joint rests against the turned up dorsal 
(outer) surface of the basals, and not against their inferior faces. The surface 
to which the “centro-dorsal” is attached, represents morphologically the 
surface of the concavity for the reception of the column in Palzeozoie Cri- 
noids; the inferior faces are those which meet the infrabasals, and in JZ. Or- 
bignyi and M. polydactylus actually support them, but in most of the Apio- 
crinidx they enclose a vacant space. If this space was filled by infrabasals, 
as we think it was in the young Crinoid, the columnar concavity and the 
upper face of the stem would be in exactly the same condition as in the 
Ichthyocrinidx, in which similar angularities occur on the upper face of the 
stem (Plate Il. Fig. 44). The upper face of the column in all Crinoids 
adapts its form to the shape of the plates to which it is attached, and if their 
suture lines are grooved, it will be correspondingly ridged. We thus believe 
that Carpenter misunderstood the structure, and that he overlooked our 
statement that it is the longitudinal angles along the column which alternate 
with the proximal ring of plates in the calyx, and not the angularities or 
ridges of the upper face. 
In Rhizocrinus the condition of the base is apparently similar to that of 
the Apiocrinide. The genus, however, was described by Carpenter t as 
* Ann. and Mag. Nat. Hist., March, 1886, p. 286. 
+ Chall. Rep. Stalk. Crin., p. 246. 
