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



preserved ; but the columns are numbered 1 to 5 to correspond with their 

 usual relative positions. In the second row a new column is introduced 

 by the pentaoon 6. Column 6 at its origin has 2 columns on the left and 

 3 on the right, as in Meloniies multiporus (plate 2, figure 2, area G). It is 

 therefore a left-handed series. The seventh column starts in the third 

 row above pentagon 6 in pentagon 7, which has a heptagonal plate, H, 

 on its right border. Column 7 at its point of origin has an equal num- 

 ber of columns on either side, as usual. In the fourth row above pen- 

 tagon 7 the eighth column originates in pentagon 8, with a heptagon, H, 

 on its left. This column at its origin has 4 columns on the left and 3 on 

 the right. The ninth and last column originates in pentagon 9 in the 

 eighth row above pentagon 8. Pentagon 9 has a het)tagonal i)late, H, 

 on its right border and has 4 columns on either side. This last ninth 

 series completes the numl)er of columns, no more than 9 being known 

 in the species. 



Comparing these two specimens of Olir/oporus danx (plate 6, figures 31 

 and 34), it is seen that tlie method of plate arrangement and introduc- 

 tion is exactly the same as that traced in the genus Meloniies. The sim- 

 ilarity is in the terminal pentagons with adjacent heptagons, the number 

 of columns on the left and right of newly introduced columns, and the 

 passage of plates dorsally into the rhoml)ic form characteristic of newly 

 introduced plates. jMeek and Worthen's figure corre.si)onds to the ideal 

 method of plate arrangement, except in having column 6 one row too 

 far to the left. Tiie other specimen shows no more variations than are 

 just what might be met with in Meloniies midtiporus. 



Oligoporus nobilis, Meek and Worthen (31), as figured by the authors, 

 does not show indications of the metliod of introduction of plates, but 

 in the text we find the statement that the interaml)ulacra are "... 

 composed of five rows [column:^] of large plates, all of which extend to 

 the disc above, while the middle one ends [begins] within about 0.65 

 inch of the oral opening below." This was evidently correctly oriented 

 by Meek and Worthen, as the genital plates are described. Therefore 

 this species as Olif/oporus danse increased the number of columns as it 

 grew dorsall3\ Its fifth column is acquired very earl 3^, but not earlier 

 than in Meloniies giganteus (plate 5, figure 21), where it begins close to the 

 oral area. This species, Oligoporm nobilis, in the whole length of the 

 figure published, does not show the addition of any new columns, which 

 would have been figured, 1 am confident, had they existed, as Meek's 

 drawings of Pala3echini are among the most accurate met with. This 

 length is a distance represented by the introduction of 21 rows of plates 

 and is a greater number of rows built without additional intercalated 

 columns of any species of Pal?eechini seen. Spine bosses of Oligoporus 

 nobilis are shown in plate 6, figure 35. 



