192 R. T. JACKSON — STUDIES OF PAL.EECHINOIDEA. 



presenting just such a form as seen in plate 2 of Melonites multiporus 

 (plate 3, figure 10). Restoring the broken border of plate 1, as is done in 

 plate 6, figure 26. we see that the two plates by their inclined faces include 

 an angle ventrally, just as in Melonites multiporus (plate 3, figure 10). This- 

 angle is apparently the space occupied by a missing plate, 1', Avhich, as a 

 stage in growth, corresponds exactly with a similar stage in Strongylocen- 

 trotiis (plate 3, figure 9), as discussed on page 144. This condition in 

 Oligoporus apparently represents a condition in which b}'' the advancing 

 edge of the peristome the ventral border of the interambulacrum has been 

 partially resorbed, so as to cut away a large part of plate 1' and induce 

 straight edges on a portion of the ventral areas of plates 1 and 2, as in the 

 figures of Melonites and Strongylocentrotiis cited. In plate 6, figure 27, the 

 ventral area of Oligoporus coreyi is restored to the condition it probably 

 had before any resorption of the interambulacrum took place. Plate 1 

 is there a large pentagonal plate filling the entire ventral area and having 

 the same position and form which the same plate had in Melonites as 

 restored in plate 3, figure 11, and in Strongylocentrotus (plate 3, figure 8)^ 

 as observed by Professor Loven. A similar form and position of this 

 first plate 1' of the interambulacrum is shown in this paper in Pholido- 

 cidaris (plate 9, figure 54), Lepidechinus (plate 7, figure 42), and Gonioci- 

 daris, after Loven (figure 3, page 234). A similar position, but from 

 mechanical reasons a different formed plate, is seen in the first plate, 1', of 

 Bothriocidaris (figure 4, page 234). 



In Oligoporus this first plate, T, from the law of alternation of introduc- 

 tion of columns should be the first member of column 1, the left adambu- 

 lacral column, as in Melonites. By its presence this single plate represents 

 a single column stage seen in the 3'oung of the whole class of Echini 

 as maintained by Professor Loven, and finds its ancestral representative 

 in adult Bothriocidaris (figure 4, page 234). The relations and importance 

 of this early single plate stage are discussed under Melonites, page 144. 



In the next row of plates alcove 2 and 3 in Oligoporus (plate 6, figure 25) 

 we find plate 3, which is hexagonal, and is the initial ])late of the first 

 column of median hexagonal plates, exactly as in Melonites. In the next 

 row pentagonal plate 4 appears, and it is the beginning of column 4. Its 

 ventral border impinges on the dorsal side of plate 3, inducing thereby 

 the hexagonal form of that plate, just as in Melonites. Column 4 at its 

 origin has two columns on the right and one on the left, which, as the 

 specimen is viewed from the inside, is the equivalent of two on the left 

 and one on the right, the normal number as viewed from the outside. 

 Three rows further dorsally the fifth column is introduced by pentagon 

 number 5, with a heptagonal plate, H, on its right side (left as viewed 

 from without). This column at its origin occupies a median position, 



