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



plate d, and like it has no spine boss. Areas Cand A are therefore seen 

 to be exactly alike ventraliy and are mutually helpful, for plate c, which 

 ia wanting in area C, is present in area A ; also plate /. which is present 

 in area C, is wanting in area A. 



Ambulacral plates are wanting in area Z), but the space they occupied 

 remains open.* In interambulacrum E there is a parallel but reversed 

 condition in the ventral row of plates as compared with areas C and A. 

 The ventral plate c of column 1 is not preserved, but its place remains 

 vacant, and it is restored as shown by the dotted lines. This plate c is 

 a small plate, being less than the upper half of a hexagon, like plate d in 

 areas C and A. The initial plate d of column 4 is present as a hexagon, 

 with its ventral border partially resorbed. The ventral plate e of column 

 3 is present as a small plate, less than the upper half of a hexagon, and 

 shows what plate c should have been. Plate /, the ventral plate of column 

 2, exists as a hexagon, with its ventral border partially resorbed. Plates 

 d and / have spine bosses in the original center of the plate, but on ac- 

 count of resorption lying nearer the ventral than the dorsal border. 



In interambulacrum G the row of ventral plates is like the same row 

 in area E. Plate c of column 1 is less than the upper half of a hexagon, 

 as in area E. Plate d of column 4 is a hexagon, with its ventral border 

 truncated. Plate e, the ventral plate of column 3, is wanting, but the 

 place for it exists in tlie specimen, and it is introduced as indicated b}^ 

 dotted lines. Plates c and e have no spine bosses. Plate /, the ventral 

 plate of column 2, in the specimen, is slightly twisted out of place, but 

 it is restored to its natural position in the figure ; its ventral border is 

 somewhat truncated. 



In ambulacrum H the plates are clearly preserved. In interambu- 

 lacrum / the ventral row of plates is wanting, so no attempt is made to 

 restore them in this figure. Ambulacrum / has the plates clearly pre- 

 served. In the specimen areas A and C have the same arrangement, 

 and in these the fourth column is right-handed. This point is discussed 

 later (page 220). In areas E and G a different but comparable arrange- 

 ment exists in the ventral row, and in these areas the fourth column is 

 left-handed. What the arrangement of area / would have been if perfect 

 may be pretty safely assumed from the relative position of plates. From 

 the position of plates in column 2 of area /it seems that the lowest plate of 

 this series must be the ventral plate of the area, as in column 2 of area G. 

 Therefore in the reconstruction, figure 44, this idea is carried out, and as 

 a consequence the other plates of the ventral row are the same as in 

 area G. 



* In making the drawing of figure 43, Mr Emerton did not have the specimen; but made his 

 figure from detailed drawings of the specimen. By an oversight he Inchided the ambulacral plates 

 in area Z), where they do not exist in the specimen. 



