150 JACKSON AND JAGGAR — STUDIES OF MELONITES MULTIPORUS. 



of plate with terminal pentagons of newly introduced columns. Close 

 to the genital plates the interambulacral plates are quite irregular in 

 form. A comparable irregularity of form in newly introduced plates 

 may be seen in any modern echinoid, as Strong ijloceiitrotas. Concurrently 

 with the change in form, passing dorsall)", there is a progressive reduc- 

 tion near the abactinal area of the number of columns (plate 3, figure 13). 

 This reduction, it seems, is to be ascribed to the progressivel}'' narrowing 

 area which the upper part of the interambulacral area presents. To 

 make a homely simile, it may be compared to a flock of sheep coming 

 through a narrow pass. The small nutnber in the pass does not mean 

 that the flock is lessening, but that no more can get through at once. 

 We take it, therefore, that the decreasing numl^er of phites at the dorsal 

 end of the interambulacrum does not mean that the number of columns 

 is dying out, but that as a mechanical matter no more plates can get 

 through this narrow area at once. An evidence of this crowding out of 

 columns is shown well in i)late 5, figure 20. Here the separation or 

 stringing out of successive i)lates of the same column is most striking. 

 The perpendicular dotted lines will serve to indicate which plates belong 

 to a given column, although separated. The fact that the full number 

 of columns may yet be traced in dissociated plates quite near the dorsal 

 terminus is brought out by the oblique line X-Y in })late 3, figure 13, 

 which bisects the separated plates of the 8 perpendicular columns ex- 

 istent in this interaml)ulacrum, and this fact is clearl}^ brought out by 

 the lines A'', F, Zm tlie diagram on page 104. This change to the rhombic 

 form near the dorsal area is also shown by plate 5, figures 20, 21, plate 6, 

 figure 31, and plate 7, figure 36. 



Proof that the crowding out of columns is due to the narrowness of the 

 area can only be obtained by getting young material, which several efforts 

 failed to procure. Still, it is possible tliat in quite young stages, where 

 the number of columns is smaller, all might continue to the dorsal ter- 

 mination without interruption, because the area would be relatively less 

 crowded with fewer columns. The crowding out is distinctly not a dying 

 out of columns, for when columns die out or cease to be continued to the 

 dorsal area it is commonly the middle or last added column which drops 

 out first in the cases observed, as shown in the dropping out of column 11 

 in the dorsal area o^ Melonites glganteas (plate 5, figure 21). The dying out 

 of columns is a change from the progressive addition or maintenance of 

 columns, and, in so far as it goes, may be regarded as a retrogressive 

 feature. This is not a common feature, as ol)servations show. In Ollgo- 

 porus missoariensis (plate 9, figure 50) a sixth column of plates exists for 

 a brief period, and in Lepldesthes looriheni (plate 9, figure 53) four columns 

 of interambulacral plates are found ventrally, but one soon drops out. 



