362 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



radials when present) as the first ambulacral of the urchins in the normal position 

 in contact with the infrabasal, which corresponds to the echinoid ocular. 



The rearrangement of the apical plates of the crinoid and the contraction of 

 the coronal ring which of necessity followed the formation of a column has brought 

 the basals (genitals) into a closed ring, cutting off the infrabasals (oculars) from 

 contact with the radials (the first plates of the ecliinoidal ambulacral series) and 

 preventing the formation between the infrabasals and the radials of the subradials 

 (the representatives of all of the ambulacrals of the urchins except the first two). 



In the case of species with a very large body, allowing of more or less sepa- 

 ration between the calyx plates, we find that an interradial series of plates, in 

 every way resembling the echinoid interradials, is formed above each basal (geni- 

 tal), while, excepting only in Cleiocrinus, the radial, instead of moving to a more 

 proximal position and occupying the gap between the basals as would naturally be 

 expected were the radial really the homologue of the ocular, remains in the usual 

 position, but becomes connected with the basal ring, much more rarely with the 

 infrabasal, beneath it by an additional plate. 



In other words, both the basals and the infrabasals maintain their primitive 

 relationship to the apical area (in the crinoids covered by the column or by the 

 central plate) just as strictly as do the genitals and the oculars, and the slight 

 deviations from the most primitive condition are exactly comparable to the similar 

 deviations on the part of the genitals and oculars; but whenever opportunity offers 

 both the basals and the infrabasals immediately give rise to series of plates which 

 correspond to the interradials and to the ambulacrals following the genitals and 

 the oculars of the urchins. 



It is comparatively rare among the crinoids to find interradials and subradials 

 developed all around the calyx; but they frequently occur in the posterior inter- 

 radius and beneath the right posterior ray, as it is in this region, where the digestive 

 tube terminates, that the phylogenetical specialization of the calyx asserts itself 

 last. 



The determination of the radial as a double plate arising through the mor- 

 phological fusion of two primarily single plates at once raises the question of the 

 correctness of the supposition, commonly accepted, that the crinoid radials are 

 really the equivalent of the echinoid oculars, which are undoubtedly single plates, 



In the echinoids we find at first a circlet of 10 plates, 5 larger alternating with 

 5 smaller, about the periproctal area; the larger are the genitals, and the smaller 

 are the oculars, the former being interradial and the latter radial (figs. 71, 72, p. 127). 



From the smaller (the radial oculars) arise the double series of ambulacrals, 

 addition to which is invariably made just under their outer border. 



The solid subspherical calcareous investment of the unattached echinoid 

 imposes no particular stress upon the circlet of 10 coronal plates until a consider- 

 able size is reached, when the weakening effect of the multiplicity of the test plates 

 must be, so for as possible, counteracted. 



This is done by the elimination, one by one, in definite sequence, of the 

 smaller plates (oculars) from the coronal ring so that the perfected arrangement 

 comes to be, as seen, for instance, in the cidarids, five large interradial genitals 



