ITS K. T. JACKSON — STUDIES OF PAL.EECHINOIDEA. 



new plate was added in the next formed row, wliile plates were added 

 in the several other adjacent columns ; when the second row was formed 

 above the rhombic plate a new plate was again added in column 9, but 

 being separated from its ventral predecessor it formed a pentagon. This 

 view appears correct from the fact, strongly brought out in these studies, 

 that accessory or adventitious plates are unknown in the Melonitida?, all 

 the plates in every specimen seen being distinctl}^ associated with others 

 in definite columns. One area, G, does not show the peculiarity of the 

 rhombic first formed plate as discussed and which is existent in 4 areas, 

 A, C, E and / (table, page 180). 



Up to the ninth column the arrangement of plates in Melonites giganteus 

 can be compared with the similar arrangement in Oligoporus danse (plate 

 6, figures 31 and 34) ; also frequently" with specimens of Melonites midii- 

 poras (plate 5, figure 20). Above this point, however, it exceeds the 

 number of columns of interambulacral plates of any other species of 

 the Melonitida?. The tenth column originates in pentagon number 10, 

 with a heptagon on its right (plate 5, figure 21). Column 10 at its point 

 of origin has 5 columns on the left and 4 on the right. Comparing this 

 with the table (page 180), it is seen that the tenth column is similar in posi- 

 tion in areas E, G and /; the position of the heptagon, however, varies, 

 being on the left in area /. The tenth column is broken awa}^ in area 

 C. The eleventh column originates in pentagon 11, with a heptagon, H, 

 on the right. This column at its point of origin has 5 columns on either 

 side. It is, therefore, as well as column 10, in its correct theoretical posi- 

 tion, as deduced from the law of growth of interambulacral areas. Turn- 

 ing to the table on page 180, it is seen that the eleventh column is want- 

 ing in one area, C. In 3 areas A, E and /, it occupies the same position 

 and terminates as in plate 5, figure 21. In area G it originates one col- 

 umn too far to the right. The position of the heptagonal plate in all 

 these areas is on the right,* its usual position in odd-numbered columns. 

 In the sixth plate, in column 11 (plate 5, figure 21), a peculiar pentagonal 

 plate, N, occurs. Tliis plate is noteworthy on account of its pentagonal 

 form, for it is not a terminal plate of a column. 



Progressing dorsally, we find in an area equal to about one-sixth the 

 whole length of the interambulacrum that there is a gradual passage from 

 hexagons to plates of a more or less rhombic form, as in Melonites multi- 

 poras (page 149). These young plates have a progressively shorter line 

 on the upper and lower sides and are relatively longer dorso-ventrally 

 than the older hexagonal plates. This change may be seen in plate 5, 

 figure 21. There is not so much drawing out and separation of the plates 

 as in the same area of Melonites multiporus (plate 3, figure 13, and plate 5, 



* Excepting area G, where, owing to imperfections, its position was not ascertained. 



