MORPHOLOGICAL PART. 57 



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 G. FornshelU, 

 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 G. Fornshelli is radial 

 throughout the whole length of the stem, that of Fentacrinus 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 Metacrims, 



Among the Reteocrinidae, also, a strictly Silurian family, we find a variety 

 of transition forms. In Reteocrinus the species from the Trenton group have 

 large infrabasals, those from the Hudson River group quite rudimentary 

 ones; while m Xenocrinus and Tanaocrimis the infrabasals are altogether un- 

 represented. The axial canal of Reteocrinus is radially disposed, that of 

 Xenocrinus and Tanaocrimis 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 dicyclic 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 Hybocrinidae as being probably monocyclic, but he regards the Hetero- 

 crinidae and Anomalocrinidae as true Monocyclica. As to Hi/hocrinus, 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. HyUcrimis 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 dicyclic pseudo-cystid genus 

 Forocrinus is so intimately related with Hyhocrinus that it is doubtful if the 

 two should not be placed in the same family. Close relations also exist 



