MORPHOLOGICAL PART. 65 
(Plate II. Fig. 4 4), being in some cases perfectly fused with that plate (Plate 
VI. Fig. 11). The condition is the same as in the two species of MWillericrinus 
in which infrabasals have been recognized, and we conclude from analogy 
that a fusion of those plates eventually took place in all groups in which the 
new stem joints are not formed directly beneath the calyx. 
The case is different among the Pentacrinidx, in which the new stem 
Joints constitute the upper part of the stem. Of the principal genera which 
have been referred to this family, one — Letracrinus — has small infrabasals 
persistent through life ; while in the other two — Pentacrinus and Metacrinus 
—no trace of them can be found in the adult. The dicyclic nature of 
Pentacrinus and Metacrinus is indicated by the orientation of the stem and 
cirri, the angles of the stem in both of them being interradially disposed, 
and the cirri radially. But what became of their infrabasals? That they 
fused with the upper stem joint, like those of the Ichthyocrinid, need not 
be considered in this family, as that would necessarily prevent the formation 
of new joints at the top. They may have been resorbed in the growing 
animal ; but it appears to us more probable, from paleontological evidence, 
that the plates gradually diminished in size, and finally disappeared alto- 
gether in the group. The structure of the Pentacrinidx is very different 
from that of the Comatule and Apiocrinide, and it appears to us that Cri- 
noids in which the upper joint of the stem is the youngest, cannot be derived 
from those in which the top of the stem is fused with the infrabasals, and the 
two groups should be widely separated. The Apiocrinidee and Comatule 
which have a centro-dorsal,—7. ¢., in which the infrabasals are fused with 
the upper stem joint,—should be placed together with, or close to, the 
Ichthyocrinide ; while the Pentacrinidx, which in many points agree with 
the structure of the Poteriocrinide and Encrinide, we think might be safely 
referred to the Inadunata Fistulata. 
It is very singular that while in Evtracrinus and Metacrinus the projections 
of the open space within the basal ring, and the axial canal of the stem, are 
radially disposed (Plate VI. Fig. 9), both are decidedly interradial in Penta- 
crimus (Plate VI. Fig. 8). This anomaly, if we may so call it, was regarded by 
Carpenter * as a proof that our generalization upon the stem is not applicable 
to the Neocrinoids. This, indeed, seemed at first to be indicated also by the 
orientation of the stem; but Bury’s discovery of infrabasals in the Comatula 
larva changed the whole aspect of the case. The discovery of these plates, 
* Bibliogr. Notices, Ann. and Mag. Nat. Hist., March, 1886, p. 287. 
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