310 CLARK! PLATES IN CRINOIDS 



In Antedon the so-called anal plate is formed at about the period 

 of development of the IBr 2 , between the two posterior radials; 

 but it is noticeable that while the radial to the left of it is of normal 

 shape, that to the right has its left side more or less cut away for 

 its reception. When the anal is lifted out from the circlet of 

 radials just previous to its resorption it is noticeable that it keeps 

 to the right of the posterior interradial area, remaining more or 

 less in contact with the right hand radial and first primibrach, 

 instead of being drawn directly upward as would be expected; 

 also the right radial is asymmetrical, more convex on the right 

 side than on the left (adjoining the anal) though after the with- 

 drawal of the anal this asymmetry quickly disappears. 



The general tendency of the anal plate of the young Antedon 

 to keep to the right of the posterior interradial area, though very 

 strongly marked, does not appear ever to have attracted atten- 

 tion; but it is nevertheless a fact of the very highest importance. 



In more primitive species, in which the five infrabasals are large 

 and equal in size, the anal appears to be formed before any of 

 the radials, occupying a position in the rhombic area between 

 the corners of the basals and orals. Soon afterward the radial 

 appears, just to the right of and in line with it, between the basal 

 and oral of that side and to the right of the vertical line dividing 

 the basals and the orals. The radial grows much faster than the 

 anal, which it gradually surrounds so that trie latter comes, to lie 

 in a deep concavity in the side of the radial to the right of it and to 

 the right of the posterior interradius, well to the right of the mid- 

 line of the posterior basal. Later this right hand radial extends 

 itself beneath the anal and the concavity becomes straightened 

 out and disappears, the anal concurrently being shoved diagonally 

 forward (toward the left) and disappearing by resorption. 



In certain fossils groups there may be traced a progessive varia- 

 tion in the position of the radianal from a primitive position 

 directly under the right posterior radial to an oblique position 

 under the lower left hand corner of the radial, and finally to com- 

 plete elimination. The position of the so-called anal in the larvae 

 of the more primitive comatulids, lying within a concavity in the 

 lower left hand portion of the radial to the right of the posterior 



