206 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The radianal is in the form of a radial beneath the right posterior radial 

 in Temnocrinus, Meristocrinus, Ichthyocrinus, Clidochirus, Cleistocrinus, Protaxo- 

 crinus. 



Thus while in the comatulids the radianal moves upward and is shoved 

 outward from the radial ring onto the disk where it disappears, in the Flexibilia, 

 though this may occur in those types in which no radianal is present in the adults, 

 there is an equally marked tendency for the radianal to retain its original position 

 while the right posterior radial grows around it, culminating in the tandem 

 arrangement of these two plates, both equally developed, with the radial in advance. 



In certain genera of Fistulata, as in some Larviformia, one, two, or three 

 radials may have a large infraradial beneath it, while in others the radianal may 

 be very much larger than the right posterior radial. 



Although occurring in a great number of diverse forms, and usually reduced 

 in size, the radianal shows two outstanding characteristics: (1) It is invariably 

 associated with the right posterior radial, and its reduplications (infraradials) 

 are equally closely associated with other radials, and (2) whenever it is large and 

 well developed it resembles a radial itself. 



This latter feature is illustrated by the following cases : 



At first in Promachocrinus the radianal and the right posterior radial are 

 exactly alike and situated side by side, forming a transverse pair of similar ossicles. 



In Temnocrinus, Meristocrinus, Ichthyocrinus, Clidochirus, and Protaxocrinus, 

 the radianal is in the form of a supplementary radial situated beneath the right 

 posterior radial and with it forming a longitudinal pair of similar ossicles. The 

 same arrangement also occurs in certain genera of Larviformia and Fistulata, often 

 on more than one ray. 



In Hybocrinus the radianal lies within the radial circlet and is practically the 

 equivalent of the other radials, but bears a small right posterior radial on its trun- 

 cate right distal angle. 



In Baerocrinus the right posterior and the anterior radials are replaced by 

 infraradials (the equivalents of radianals) which resemble the other radials 

 except that they bear no arms. 



Since whenever the radianal is sufficiently developed to exhibit any distinctive 

 features at all it always exhibits the characteristics of radials, and since it is 

 always intimately associated with the right posterior radial, in the light of the 

 conditions seen in the very young Promachocrinus, and in Temnocrinus, Meristo- 

 crinus, Ichthyocrinus, Clidochirus, and Protaasocrwius, the assumption is justified 

 that it is in reality the equivalent of the right posterior radial. 



In the comatulids the gradual and eventually complete elimination of the inf ra- 

 basals and basals as elements of the wall inclosing the visceral mass and the disap- 

 appearance of the orals, a morphological transformation which runs hand in hand 

 with the extraordinary prolongation and development of the arms, is easily traced 

 in the ontogeny, and among the fossil forms we find evidence supporting the 

 belief that phylogenetic advance has in general been along similar lines, involv- 

 ing a reduction in the number and size of the calyx plates correlated with an 

 increase in the length and size of the arms. 



