104 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
but covering pieces, which gradually in geological time changed their 
character. . 
The “radial dome plates” of the Actionocrinide and allied forms are 
generally larger than any of the surrounding plates, often nodose, and some- 
times extended into long spines. They are not followed immediately by 
covering pieces, as already stated, and are placed at some distance from 
the orals, — occupying in the simpler forms with but two arms to the ray 
almost the outer margins of the tegmen (Agaricocrinus), — directly over the 
point at which the bifurcation of the ambulacra takes place. When there are 
four arms to the ray, they are removed relatively further inward, and are 
followed by two similar but smaller plates of higher rank. But when there 
are three arms to the ray, there is only one such plate, which is directed to 
the side where the bifurcation is, the opposite side of the plate being followed 
by the regular covering pieces of the arms. 
From this structure we may infer that the so-called radial dome plates 
with subtegminal ambulacra are axillaries, and if they represent, as we have 
reason to believe, modified covering pieces, that they are the plates from 
which the ambulacra bifurcate. In this view it is quite suggestive that the 
axillary plates of the ambulacra are frequently protuberant. In Lueladoeri- 
nus millebrachiatus (Plate LX XIII. Fig. 1), they are all along the main arms 
strongly nodose, and if the ambulacra of this species had been covered by 
other plates, the tips of the axillaries naturally would project above them 
and be exposed upon the disk. In this way the radial dome plates may have 
originated, so that afterwards the upper portions developed to larger size, and | 
finally become independent plates. This explanation seems to us most prob- 
able, and it was favorably received by Carpenter. 
The Palxontological evidence indicates that in the earlier Camerata, as in 
the young specimen at some time, the ambulacra were exposed upon the 
disk. In most of the Silurian forms they took part in the tegmen, and their 
covering plates, as a rule, were more regular in their arrangement than in 
those of later epochs. In the Carboniferous, with the exception of the 
Platycrinide, Hexacrinide and Acrocrinide, the ambulacra are almost 
exclusively subtegminal, and the whole disk assumes that extravagant 
form which led at one time to the belief that it represented an entirely 
different structure. 
