2), consists of five large triangular plates, alternating with the radials, and called "orals," because they roof over the mouth.
Bury himself, however, has inflicted a severe blow on the theory by his proof that the so-called oculars of Echinoidea, which were supposed to represent the radials, are homologous with the "terminals" (i.e.
Those plates are perhaps constant throughout sea-urchins and starfish (though it would puzzle any one to detect them in certain Silurian echinoids), and they may be traced in some of the fixed echinoderms; but there is no proof that they represent the radials of a simple crinoid, and there are certainly many cystids in which no such plates existed.
These radials may be homologous with the terminals already mentioned, but this is neither necessary nor certain.
- Blastoidea in which the thecal plates have assumed a definite number and position in 3 circlets, as follows: 3 basals, 2 large and I small; 5 radials, often forkshaped, forming a closed circlet; 5 deltoids, interradial in position, supported on the shoulders or processes of the radials, and often surrounding the peristome with their oral ends.
The stereom of the radials and deltoids on each side of the ambulacra is thrown into folds, running across the radio-deltoid FIG.
The brachia rise from a corresponding number of thecal plates, "radials (RR)."
Below these is always a circlet, or traces of a circlet, of plates alternating with the radials, i.e.