548 HERKICK /•:. WILSON 



that genus and restoring the orals an ideal larval or ancestral 

 form will appear. It. however, the anal plate is a primary plate 

 in the radial cycle, a different line of descent is indicated. 



In the ontogenetic development of the skeleton in the living 

 Antedon the anal plate is interpolated after the basals and orals 

 have formed closed cycles, and before the radials are laterally in 

 contact. As the anal expands by lateral and proximal growth, it 

 comes into contact with the posterior radials and the posterior 

 basal; but this happens before the distal margins of the basals 

 are completed and while the radials are still separated from each 

 other. Since growth in the anal plate and the posterior radials 

 does not cease upon their coming into contact, shoving of the 

 posterior radials in the direction of the unoccupied lateral areas 

 might be expected. This, however, does not occur. On the con- 

 trary, the crowding results in a partial inhibition of growth in the 

 apposed margins, and a marked asymmetry results in the outline 

 of the posterior radials, especially in the right-posterior radial. If 

 comparison is now made between Antedon in this stage of develop- 

 ment and the early Camerata having a hexagonal base, a striking 

 similarity is seen in the development of the basals, radials, and the 

 posterior side of the calyx. The posterior radials in both forms 

 are asymmetrical and narrower than the anterior radials, and 

 the asymmetry is due to the diminution of the posterior side of the 

 plates, the distance between the center of the radial facet and the 

 plate margin being less in the posterior half than in the anterior 

 half. If comparison is then made between the relative position of 

 axial lobes and radial plates in pentagonal and hexagonal Camerata, 

 a further distortion is noted in the hexagonal forms. The lobes of 

 the canal in the pentagonal forms occupy an interradial position, 

 while either two or three of the lobes in the hexagonal forms occupy 

 a radial position. These facts show that there is a distorting factor 

 present in the posterior side of the cup. When it is considered 

 that pentamerous symmetry is the rule in Echinoderms, and that 

 sutural symmetry based upon the hexamerous plan only appears 

 in the basal cycle with the appearance of the posteriorly 

 directed basal suture, there seems to be no other alternative 

 than that the anal plate is the distorting factor, and that it has 



