174 E. T. JACKSON — STUDIES OF PAL.EECHINOIDEA. 



as is the common condition in specimens of Melonites. From the small 

 size and irregular outline of the plates, their contours are largely de- 

 stroyed by the process of silicification. Starting with 4 plates at the 

 ventral end (plate 5, figure 22), new plates and new columns are pro- 

 gressively added, as in Melonites midtiporus^ (plate 2, figure 4), until in 

 the middle of the corona 12 columns were observed. At this region no 

 area was sufficiently clear to be figured, so that in representing the am- 

 bulacrum (plate 5, figure 24) I was obliged to select an area considerably 

 below the median zone which had not acquired the full complement of 

 6 columns characteristic of each half-ambulacrum in later stages. The 

 portion selected for the detailed figure of the ambulacrum is indicated in 

 plate 4, figure 19, by the letter X in area B. At the area marked by Y, 

 just above this point, by close inspection the outline of 6 plates may be 

 made out in this half-ambulacrum. 



It is possible that a specimen more perfect in ambulacral detail might 

 show still more columns added in a higher zone. The plates of the two 

 median columns of the ambulacra a' h' are very large (plate 4, figure 

 19, and plate 5, figure 24), and the pores in these, as in other plates of 

 the area, exist in the part of the plate lying nearest the interambulacra^ 

 as in Melonites multiporus. All the plates observed possess two pores. 

 The plates of the ambulacra and interambulacra are thickly studded 

 with tubercles that formed the base of attachment of spines (plate 5 

 figure 23; also seen in plate 4, figure 19). There are about 25 such 

 tubercles on the larger interambulacral plates, but no spines are pre- 

 served. There are obscure traces of genital and ocular plates, but no 

 details of form or structure were made out. 



The interambulacrum of Melonites giganteus is interesting as a study on 

 account of the great numl^er of plates and columns existent and for a 

 comparison of the arrangement of the same with other Paheechinoids, 

 The most perfect area in the specimen, which by the adopted notation is 

 designated as A, is illustrated by Mr Emerton's very skillful drawing in 

 plate 5, figures 21 and 22. It is also well shown up to and including the 

 introduction of the ninth column in plate 4, figure 19. The arrangement 

 of this same area is also represented, together with the arrangement of 

 the other 4 interambulacral areas, as far as they can be ascertained, in the 

 table (page 180). This table is a graphic proof of the perfection of the 

 specimen, as in it we are able to tabulate the essential details of no less 

 than 42 columns of plates throughout their entire length. In many por- 

 tions, where the surface was eroded, the form of the plates couki yet be 

 ascertained by the middle, or lower proximal portion of the plates, which 



*0n account of imperfections from silicification the transition is not shown so clearly as in the 

 fio-ure cited, but enough is visible to prove the similar method of introduction. 



