202 ^' ^- J-^CKSON — STUDIES OF PAL^ECHINOIDEA. 



on either side and a heptagonal plate on the left (right as viewed from 

 without). Six rows above pentagon 7 the eighth column is introduced 

 by pentagon 8, with a heptagonal plate on its right. Column 8 has 3 

 columns on the left and 4 on the right, these and the heptagonal plate 

 being in the correct position when reversed for viewing from the outside. 

 Ambulacrum B, as the other ambulacral areas, has crenulations on the 

 sides of the area and a median crenulation corresponding to the outer 

 and median borders of two columns of low ambulacral plates. On ac- 

 count of the small size of the ambulacral plates and the relative coarse- 

 ness of the sandstone cast, the outlines of the plates cannot be farther 

 determined, but they are indicated by restoration in the dotted lines con- 

 necting the crenulations in ambulacrum /. On account of the condition 

 of preservation, I could not sa}^ whether or not the plates of each half 

 ambulacrum lapped over one another. Meek and Worthen's figure of 

 the species, however, cited above and here copied in i)late 7, figure 37r 

 shows the ambulacral plates slightly overlapping, much as in Rhoechinus 

 elegans (plate 7, figure 40). Interambulacrum Cat the lower border, as 

 far as preserved, has 7 plates, representing 7 colunnis. The eighth column 

 is introduced by pentagon 8, with a heptagonal plate on its right. 

 Column 8 at its origin has 3 columns on tlie left and 4 on tlie right. This' 

 relation and the position of the heptagon are normal when reversed for 

 comparison with an outside view of the area. Area Chas a ninth column 

 introduced by pentagon 9, with a heptagon on the left (right as viewed 

 from without) and 4 columns on eitlier side. This ninth column is prob- 

 ably to be considered an exceptional addition. No species of Rhoechinus 

 has previously been described as having 9 columns. In interambu- 

 lacrum E there is a row of 7 plates on the ventral border as far as pre- 

 served. The eighth column is introduced by pentagon 8, with 4 columns 

 on the left and 3 on the right (the incorrect position when reversed for 

 comparison with the outside). Next to pentagon 8, on the left, is an 

 octagonal plate, 0, corresponding to a heptagonal plate (and in the incor- 

 rect position when reversed for comparison with the outside). A hep- 

 tagonal plate exists at 0', and two tetragonal plates at T T; also a hei)- 

 tagonal j^late exists at H. The angles of these plates compensate in part 

 for the want of one side in pentagon 8 and the want of two sides in 

 tetragons jTand T. A similar compensation of sides is seen around pen- 

 tagon 9 in Melonites giganteus (plate 5, figure 21). Interambulacrum G 

 has a row of 7 plates on the ventral border as far as preserved. The 

 eighth column is introduced by pentagon 8 with a heptagon on its right 

 and with 3 columns on the left and 4 on the right, the normal position 

 when reversed for comparison with the outside. Interambulacrum lis 

 similar to G, except that column 8 and its adjacent heptagon are one 



