146 JACKSON AND JAGGAR STUDIES OF MELONITES MULTIPORUS. 



onal plates, as seen by following the dotted lines in the figure. Initial 

 plate 3 is hexagonal in form ; but, comparing it with the initial plates 

 of later added columns (plate 2, figure 2), it will be seen to be similar in 

 form ventrally and on the two sides. It is potentially pentagonal, if we 

 may be allowed the expression, but is hexagonal because its dorsal border 

 is truncated by the impinging ventral border of the terminal pentagon, 

 number 4 of column 4. The terminal pentagon always induces by im- 

 pact an additional side on one of its ventrall}^ bordering plates, as seen 

 in later added columns, and in this case it thus makes a hexagon out of 

 a plate normally pentagonal. Plate 3 is typically hexagonal, and ex- 

 ceptions to this form are very rare. Column 3 at its point of origin is 

 obviousl}" central in position, having a column of lateral adambulacral 

 plates on either side. In the next row dorsally there are four plates, 

 pentagon number 4 being the initial plate of column 4. The ventral as- 

 pect of this pentagon impinges on plate 3, causing it to take on a hexag- 

 onal form as just discussed. In four areas of the figure, A, C, E and G, 

 column 4 at its point of origin has two columns on the left of it and one 

 on the right ; in a fifth area, I, the reverse obtains, there being one column 

 on the left and two on the right. This shows at the start a variation 

 which is not infrequent in even-numbered columns. Another case of the 

 same kind is shown in the introduction of column 6 in area G of the same 

 figure. Here there are two columns on the left and three on the riglit, 

 whereas in the two other areas, E and I, sliowing the introduction of the 

 sixth column, there are three columns on the left and two on the right. 

 While even-numbered columns in most cases originate to the right of the 

 center, having one more column on the left than on the right, they some- 

 times originate to the left of the center, having one more column on the 

 right than on the left. Other examples of this variation are seen in 

 plate 3, figure 10, and in the tables on pages 165 to 170, inclusive. 



Later stages, with new columns added after the fourth, might be traced 

 in plate 2, figure 2, but they can best be followed by comparison with the 

 description of stages represented by figure 12 of plate 3. It will be seen 

 in plate 2, figures 2 and 3, that columns 3 and 4 are introduced ver}^ 

 early in passing from the ventral area dorsally. This rapid introduction 

 is also shown well in Melonites glganteus, sp. nov. opiate 5, figure 21). It 

 is the common condition, but exceptions may be found, as in plate 3, 

 figure 12, where column 4 originates much later than column 3. The 

 rate of introduction of new columns, as shown in plate 2, figure 2, may 

 be accepted as the average rate in Melonites maltlporics, to which we have 

 seen but two exceptions, namely, those shown in plate 3, figures 1^ and 16. 

 The rapid introduction of the new columns 3 and 4, in plate 2, figure 2, 

 shows an accelerated development which is still more accentuated in 



