DESCRIPTION OF MELONITES GIGANTEUS. 179 



figure 20). This may be a specific or simply individual difference, or 

 even a difference due to the age of the individual. There is a very dis- 

 tinct dropping out of the middle and last added column, number 11. 

 This column is reduced to a tiny rhombic plate at its upper limit, and is 

 not represented at all in the last rows built by the echinus as far dorsally 

 as they can be traced. On the other hand, the last (eighth) column in 

 Melonites multiporus extends directly to the genital plate (plate 3, figure 

 13). It is an axiom of old age characters that the last features acquired 

 are soonest lost, and it seems that this specimen of Melonites giganteus 

 had entered on its decline, which is shown by the dropping out of a 

 column that at a little earlier period in growth must have been continu- 

 ous to the genital area. During later growth, had such taken place, this 

 last column apparently would not have been existent, so that the indi- 

 vidual in its decline has virtually resumed the condition of building only 

 10 columns of plates, a feature which was initiated at a relatively early 

 age, as shown by pentagon 10, plate 5, figure 21 (see page 150). There 

 is a distinct inequilaterality of the interambulacrum near the dorsal area, 

 as shown in plate 5, figure 21. Columns 9 and 10 are quite alike on the 

 two sides of the center, but columns 7 and 8 are, on the contrary, very 

 -unlike, as are also columns 5 and 6. 



In each of the 5 interambulacral areas of Melonites giganteus the several 

 columns, as demonstrated in the fourth column of the table (page 180), 

 made their appearance at exactly or nearly the same horizon. The great- 

 est exactitude in the period of introduction is maintained in the earliest 

 added columns, especially numbers 3, 4, 5, 6, and 7, while in the columns 

 appearing last there is more variation, as might be reasonably expected. 

 Stating the case briefly, in all areas the third column originates in the 

 second row and the fourth column in the third row, as in Melonites multi- 

 porus. The fifth column originates in the fifth or sixth row ; the sixth 

 column in the eighth row ; the seventh column in the tenth or eleventh 

 row ; the eighth column originates in the twelfth to fourteenth row ; the 

 ninth column originates in the sixteenth or seventeenth row ; the tenth 

 column originates in the twentieth to twenty-third row, and the eleventh 

 column originates in the twenty-fifth to the twenty-ninth row. Compar- 

 ing this result with that shown in Melonites multiporus, where it has been 

 ascertained in a large number of specimens (page 162), we find that after 

 the fourth column each of the later added columns originated much 

 earlier in Melonites giganteus than in Melonites multiporus. This species, 

 therefore, is not only furthest advanced in the special line of variation of 

 the genus, but it is also highly accelerated in its development, very early 

 passing through those stages seen in later growth in less specialized mem- 

 bers of the genus. 



