MORPHOLOGICAL PART. 67 
The case of Glyptocrinus Fornshelli is very remarkable. The orientation 
of the stem points to a monocyclic base, that of the axial canal to a dicyclic. 
Besides, the species is closely related to others in which the canal is inter- 
radial, and the stem radial. It is quite evident that the rule which governs 
the relations of the parts below the base does not hold good in @. Fornshelli, 
as we find it in the fossil state; but we think this proves nothing more than 
that in this species the monocyclic stage was as yet incompletely developed. 
It should be stated that while the aberrant canal of @. Fornshelli is radial 
throughout the whole length of the stem, that of Pentaerinus is interradial 
only at the upper portions; it soon turns to circular, and where the cirrus 
vessels enter, it is as radially disposed as that of Metacrinus. 
Among the Reteocrinids, also, a strictly Silurian family, we find a variety 
of transition forms. In efeoerinus the species from the Trenton group have 
large infrabasals, those from the Hudson River group quite rudimentary 
ones; while in Xenoerinus and Tunaocrinus the infrabasals are altogether un- 
represented. The axial canal of Reteocrinus is radially disposed, that of 
Xenoerinus and Tunaocrinus interradially. The three forms are very closely 
related, and if we were to separate them upon the structure of the base, it 
would be to the exclusion of other characters of manifest importance. 
The examples given indicate that there is a most intimate relation 
between dicyclic and monocyclic forms, and that probably the latter were 
derived from the Dicyclica by a gradual decrease in size and final oblitera- 
tion of the infrabasals in geological time. 
Bather believes with us that dicyclie Crinoids preceded the monocyclic, 
but he states that so far as the Fistulata are concerned he has found no geo- 
logical evidence to prove it, at least not among the Fistulata. He alludes to 
the Hybocrinidx as being probably monocyclic, but he regards the Hetero- 
erinide and Anomalocrinide as true Monocyclica. As to Hybocrinus, he quotes 
the earlier part of the Revision, in which we stated that rudimentary infra- 
basals might possibly be present in the genus. This supposition is not veri- 
fied by further study of the specimens, which show satisfactorily that no 
such plates are represented. Hybocrinus is a true monocyclic genus, as much 
so as Heterocrinus and Anomalocrinus. This, however, does not exclude the idea 
that all three forms may have been derived from the Dicyclica, and there 
are good reasons to believe that it was so. The dicyclie pseudo-cystid genus 
Porocrinus is so intimately related with Hyboerinus that it is doubtful if the 
two should not be placed in the same family. Close relations also exist 
