68 ROBERT TRACY JACKSON ON ECHINI. 



In Melonechinus muUiporus there are eight or nine, rarely seven, columns of plates in each 

 area. In M. vanderbilti there are nine columns of plates, and in M. giganteus (Plate 59, fig. 14) 

 there are eleven. The columns are all added in a perfectly definite method and are in this 

 feature further developments of the system seen in Palaeechinus quadriserialis. 



The progressive addition of columns as we pass dorsally from the single column ventrally 

 to the two to fourteen columns dorsally, I believe, represents distinctly stages in development 

 through which the animal passes in its growth, the progressive stages being comparable to the 

 conditions seen in the adult of simpler genera or simpler species allied to the type under con- 

 sideration. 



The method of introduction of interambulacral columns was shown in detail by Jackson and 

 Jaggar (1896) and Jackson (1896), but will be briefly stated here. In the Palaeechinidae and 

 most Palaeozoic Echini, excepting those that have strongly imbricate plates, the plates of the 

 interambulacrum are typically hexagonal excepting the adambulacral columns which are nearly 

 pentagonal but crenulate or rounded on the adradial suture. WTiile the plates are typically 

 hexagonal, this symmetry is modified where new columns are introduced. The initial plates 

 of columns above the third are typically pentagonal with an apex of the pentagon pointing 

 ventrally, and an adjacent ventral plate on the left or right is heptagonal, the added side com- 

 pensating for the one side short in the initial pentagon. An exception to this rule of pentagonal 

 initial plates of columns is seen in the initial plate of the third column, which is typically hex- 

 agonal and is immediately succeeded by the initial plate of column 4, which is a pentagon, as 

 seen in Melonechinus muUiporus (Plate 57) and many other species. If by rare exception the 

 initial plate of column 3 is not immediately followed by the initial plate of column 4, as in 

 area C of Lovenechinus septies (Plate 45, fig. 1), then the initial plate of column 3 is also pen- 

 tagonal. When odd-numbered columns, as columns 3, 5, 7, 9, 11, etc., are introduced, they 

 are typically introduced in the middle of the area with an equal number of columns on each 

 side of them, and also typically the heptagonal associated plate lies on the right ventral border 

 of the initial pentagon. When even-numbered columns are added, as columns 4, 6, 8, 10, etc., 

 they are typically added to the right of the center with one more column on the left than on 

 the right. While this is the rule in the large majority of cases, there are frequent exceptions in 

 which even-numbered columns are added on the left instead of the right of the center. As a 

 concurrent character, the associated heptagonal plate in even-numbered columns typically lies 

 to the left of the initial pentagon. Why this systematic position of columns and associated 

 heptagonal plates should come in with such regularity is not obvious, but it is a very constant 

 character, as shown by Jackson and Jaggar {1896, p. 163) who worked out the percentages of 

 normal cases and variations. In llie dorsal portion of the interambulacrum the plates are 

 nearly or quite rhombic in outline and gradually take on the typical hexagonal outline, as 

 they are pu.shed ventrally by later intercalated plates and come into mechanical contact with 

 adjacent plates of their several associated columns (Plate 59, fig. 14). 



