204 BULLETIN 82, UNITED STATES NATIONAL. MUSEUM. 



Its identification in the pentacrinoids has been delayed by the fact that in 

 Antedon it does not appear until relatively late, when it occupies a position between 

 the two posterior radials, the left side of the right posterior radial being cut away 

 to accommodate it. 



In this genus its position is essentially that of the so-called anal x in the 

 fossil forms with which it has been confused. But its relation to the right posterior 

 radial is that of the radianal and not that of anal x, it moves outward from the 

 radial circlet exactly as does the radianal in other types, and anal x, though 

 greatly reduced, is usually also present. 



In Promachocrinus (see pp. 530-557) and in certain other types the radianal 

 is formed before the first appearance of the radials, so that following the so-called 

 prebrachial or cystid stage a stage occurs in which the radianal is the only radially 

 situated plate in the calyx; but almost immediately the right posterior radial 

 appears as an exactly similar plate situated just to the right of it and in the 

 same transverse plane. During subsequent growth the radianal moves upward 

 and toward the left until it is nearly over the apex of the posterior basal, when, by 

 the approximation of the two posterior radials beneath it, it is shoved directly 

 outward onto the disk and disappears. 



The radianal in Antedon is not formed until a stage in the general develop- 

 ment is reached equivalent to that in which in Promachocrinus the radianal has 

 reached a position near the apex of the posterior basal. 



In the comatulids until the final extrusion of the radianal from the radial 

 circlet the centers of the radianal and of the right posterior radial always lie in 

 the same transverse plane, the radianal moving upward and outward as the radial 

 increases in size; but on account of the shape of the radial the radianal often 

 appears to lie more or less beneath its lower left-hand border. 



In the Flexibilia there is the following diversity in the occurrence of the 

 radianal: 



The following genera are without a radianal : Mespilocrinus, Calpiocrinus, 

 Harmocrinus, Nipterocrinus, Lithocrinus, Metichthyocrinus, Synaptocrinus, Euryo- 

 crinus, Amphicrinus, Dactylocrinus, Synerocrinus, Wachsmuthicrinus, Parichthyo- 

 crinus. 



The radianal may be situated obliquely above the right posterior radial, pushed 

 upward on the right shoulder of the elongated posterior basal, in Forbesiocrinus, 

 EutaxocrinuSj Taxocrinus, Onychocrinus in the last three being the first plate of 

 the tube series, usually larger than those following. 



The radianal is rhombic and situated obliquely below the right posterior 

 radial in Lecanocrinus, Cholocrinus, Pycnosaccus, Gnorimocrinus. 



The radianal is obliquely below the right posterior radial, usually between 

 the basals, in Sagenocrinus. 



The radianal is more or less beneath the right posterior radial, but above 

 the line of basals, in Anisocrinus. 



The radianal is beneath the right posterior radial and between the basals 

 in Homalocrinus. 



