66 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
whose presence we had inferred from the orientation of the larval stem and 
centro-dorsal, showed that our observation must be one of wide signifi- 
cance. The aberrant structure of the canal in Pentacrinus does not invalidate 
the law as we have found it, but simply points to the existence in some 
groups of transition forms intermediate between Monocyclica and Dicyclica. 
Such transition forms must have occurred at some time in the developmental 
history of the two groups, if one was evolved from the other. Which form 
is the older has not been satisfactorily proved, but the evidence of Palzontol- 
ogy points to the Dicyclica as the ancestral type. In the Camerata the evo- 
lution was apparently complete at and before the Silurian, but it is probably 
still going on in some of the later groups. In the Pentacrinide, the diminu- 
tive size of the infrabasals in Eztracrinus may be the first step toward the 
monocyclic base, their non-representation in Metacrinus and Pentacrinus the 
next, and the change in the orientation of the axial canal another important 
step in that direction. 
We have discovered a case almost parallel to that of the Pentacrinide in 
the Lower Silurian monocyclic genus Glyptocrinus, which has a radial stem * 
and an interradial canal, except in @. Fornshelli, in which canal and stem 
both are radial (Plate XXI. Fig. 5). 
Glyptocrinus belongs to a series of monocyclic and dicyclic Crinoids, which 
are so closely intermingled and intimately related that it is extremely difficult 
to separate them generically, and one is inclined to place in the same family 
monocyclic and dicyclic forms. There is perhaps no other group so likely to 
throw light upon the derivation of the Monocyclica. The base of Glyptocrinus 
has been variously described as consisting of one or two rings of plates. Hall 
originally defined the genus as having basals only, but a few years later 
thought he had discovered within the basal ring in some of the species indi- 
cations of five additional pieces, which were also observed by Meek, and 
called by him “sub-basals” (Plate VI. Fig. 12). §S. A. Miller described the 
base as consisting of but one ring of plates, but he included in the genus 
several species with two rings. We described the genus as dicyclic in Part 
II. of the Revision, but in Part III. placed it among the Melocrinide, after 
throwing out those species in which rudimentary infrabasals could be satis- 
factorily traced. 
* The nucleus of the stem in G. decadactylus and G. Dyeri is obscurely pentangular at the upper end ; 
the projecting edges of the joints, however, give it a circular outline. The axial canal in both species is 
sharply stellate. 
