JO SMITHSONIAN INSTITUTION 



his Stage I (pi. 9, fig. 1), in which the calyx consists only of basals and orals 

 with no trace of radials, to Stage IV (fig. 4), in which the radials and brachials 

 have become well established, and the anal plate is distinct — cutting into the 

 left side of the radial by a curved border; and finally to Stage V (fig. 5) in 

 which the radials have formed a closed ring and the arms have begun to 

 branch. It is clearly shown by this independent material of an additional 

 species of Antedonid, living remote from those previously investigated, that 

 the so-called anal plate in that genus does not appear until after the radials are 

 well under way, and then in a concavity at the left side of the right posterior 

 radial. This is in complete agreement with the order of appearance of the 

 plate as seen by W. B. Carpenter, Thomson, and Sars in two other species of 

 the Antedonidae; and it is therefore probably the general rule in the family. 



The most recent investigations upon the pentacrinoid larva of the 

 comatulids are those of Mr. Austin Hobart Clark, whose extensive researches 

 upon the Existing Crinoids, embracing collections of world-wide distribution, 

 have given him unprecedented facilities for observation of structural details in 

 specimens at different stages of growth. The results of his studies upon the 

 so-called anal plate in the Recent forms throw important light upon its position 

 and relations. 1 He has examined " many hundreds of pentacrinoid larvae be- 

 longing to numerous species distributed in several families." Based upon 

 these observations he gives the following account of the course followed by the 

 anal plate during successive stages (quoting from the monograph, p. 331) : 



In Ante don the so-called anal plate is formed, at about the period of development of the 

 IBr,, 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 withdrawal 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 attention ; but it is nevertheless a fact of the very highest importance. 



In the young of Promachocrinus, in which the five inf rabasals 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 orals. The radial grows much faster than the anal, 

 which it gradually surrounds, so that the 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 midline of the posterior basal. Later this right hand radial extends itself beneath the 



1 On the Homologies of the so-called Anal and other plates in the pentacrinoid Larva of the free 

 Crinoids, Jour. Washington Acad. Sci., vol. 2, 1912, pp. 309-314; Monograph of the Existing Crinoids, 1915, 

 PP- 331-339. 



