106 CLEIOCRINUS. 



remembered that this Crinoid occurs in the lowest part of the Ordovician, or 

 Lower Silurian, contemporaneous with the Cystids; and it may well be that 

 it represents the end of some line of evolution in which the alternate succes- 

 sive arrangement of the skeletal elements has not yet been fully established, 

 — a consideration which may also be borne in mind in connection with the 

 abnormal orientation of the axial canal. But the fact that these plates of 

 the exterior circlet, hanging to the outside of the calyx as it were without 

 visible office, should have just the angular faces to enable them to fit into 

 their proper places in the normal succession, seems to me most wonderful. 



Yet this case may not, after all, be much more anomalous than the con- 

 verse one of the Upper Silurian genus Calpiocrinus, where the infrabasals, in- 

 stead of being within the circlet of basals, have overgrown them to such an 

 extent as to completely envelop and conceal, not only the basals, but even 

 sometimes the radials also. 



SYSTEMATIC RELATIONS OF CLEIOCRINUS. 



The prime fact suggested by these observations on the actual specimens 

 is that Cleiocrinus is evidently an intermediate form between the Camerata 

 and the Flexibilia, of a most interesting character. So far as the calyx is 

 concerned, the general habitus of the specimens is that of an Ichthyocrinus ; 

 its articulate structure and flexible calyx point strongly toward the Flexibilia. 

 On the other hand, the presence of pinnules, and of five infrabasals instead 

 of three, differentiate it absolutely from the known Flexibilia Impinnata, to 

 which all palaeozoic forms of the group hitherto known belong. The bra- 

 chial structure — the form and arrangement of the arms and pinnules, with 

 ridges following the radial lines and running into the arms — is, as already 

 suggested, essentially that of Gb/jitocrinus or Reteocrinus. The superficial 

 resemblance, in this respect, is perhaps greater toward such a form as Glyp- 

 tocrinus dyeri (PI. L, Fig. 13), but structurally the analogy is much closer 

 with Reteocrinus ; — although I do not mean to be understood as claiming 

 that the two are actually at all closely related. 



If we eliminate the interbrachial system from Reteocrinus, all except the 

 anals, we shall have substantially the calyx of Cleiocrinus. The dicyclic base 

 with five infrabasals, the longitudinal series of anal plates extending almost 

 to the margin of the disk, and the incorporation of pinnules within the calyx 



