540 HERRICK E. WILSON 



perisomic 1 interbrachials, of the ambulacrals between the orals and 

 interambulacrals, and of the peculiar plates appearing between 

 the basal and radial cycles of Acrocrinus. 2 Interpolation in the 

 basal cycle is known in but two genera. In Sagenocrinus and 

 Homalocrinus the radianal is incorporated in the basal cycle, and in 

 Sagenocrinus especially it assumes the appearance of a basal plate. 

 Interpolation in the basal cycle, being well established in at least 

 two instances, seems very rare, yet this may be due entirely to our 

 lack of knowledge, for the evolution of the radianal plate in the 

 Flexibilia leads one to believe that it appeared in its primitive 

 state in the basal cycle. 3 A discussion of interpolation in the 

 radial cycle has been purposely omitted in the preceding citations, 

 as it belongs more properly in a discussion of the origin of the anal 

 plate, where it will be fully considered. 



It appears then that every cycle of plates excepting the infra- 

 basals, and every series excepting the brachials, are affected by 

 interpolation, and even in the brachial series Clark 4 has evidence 

 which points strongly to interpolation. There is then a possibility 

 that every plate cycle and series may be subject to interpolation, 

 but this point is not of immediate consequence and need receive 

 no further attention. 



Interpolation in the calyx may demand (i) a reduction in some 

 adjacent plate or plates of the same cycle or group, with or without 

 oblique development of the plates, 5 as in the reduction of the 

 posterior radials and oblique development of the radial cycle in 

 Antedon upon interpolation of the anal plate; (2) an increase in 

 diameter of some adjacent plate or plates in an apposed cycle or 

 group, with or without trunkation, depending upon the alternating 

 or superimposed position of the interpolated plate, as in the enlarge- 

 ment and trunkation of the posterior basal in some of the Camerata 



1 Ref. 16, p. 339. 



2 Ref. 39, pp. 805-10, PI. 53, Figs. 1-3, 4-9, and 10a, b. 



3 Ref. 31, p. 493, PI. 5, Fig. 9. 



A Ref. 13, p. 119. 



5 Oblique development is here used in reference to the displacement of either the 

 proximal or distal ends of a plate, from the vertical axis of the cup and not from 

 the planes of pentamerous symmetry. 



