66 THE CRmOIDEA CAMERATA OF NORTH AMERICA. 



whose presence we had inferred from the orientation of the larval stern 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 Palseontol- 

 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 Pentacrinidae, the diminu- 

 tive size of the infrabasals in Extracrinus may be the first step' toward the 

 monocyclic base, their non-representation in Metacriniis 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 Pentacrinidse in 

 the Lower Silurian monocyclic genus Glf/ptocrinus, which has a radial stem^ 

 and an interradial canal, except in G, 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 difiicult 

 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 Gli/ptocrimis 

 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 ^YQ 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 Eevision, but in Part III. placed it among the Melocrinidge, after 

 throwing out those species in which rudimentary infrabasals could be satis- 

 factorily traced. 



* The nucleus of the stem in G. decadactylus and Q. 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. 



