426 ROBERT TRACY JACKSON ON ECHINI. 



laterally bevel under the adradial plates. In the smaller specimens the plates are rhombic in 

 outline (Plate 69, figs. 1-6; Plate 70, fig. 1; Plate 71, fig. 1), the individual plates being not 

 at all or very slightly truncated by the succeeding dorsal or ventral plate of their own vertical 

 series. As a result, the plates appear to lie in diagonal series instead of in the usual vertical 

 series. In large specimens, however, the plates by horizontal growth have taken on a hexag- 

 onal form in which the dorsal and ventral borders truncate or are truncated by the next suc- 

 ceeding dorsal or ventral plates of their own vertical series (Plate 69, figs. 7, 8; Plate 70, fig. 3). 

 It is to be observed that a specimen, as Plate 69, fig. 8, in which the plates are hexagonal at the 

 mid-zone, dorsally has rhombic plates, these younger plates as a localized character being like 

 the rhombic plates at the mid-zone in a younger individual. This is the same relative struc- 

 ture as is shown in the ambulacra of Lepidesthes wortheni (p. 417). The pore-pairs in each 

 plate lie in the dorsal half, as shown by Dr. White, and when the plates are rhombic, lie in 

 about the middle line; but as the plates widen to a hexagonal form, the pores come to lie 

 nearer the next adjacent interambulacrum than the middle of the plate, as usual in Echini 

 (text-figs. 4-14, p. 54). The ambulacral plates bear small secondary tubercles and spines 

 similar to those of the interambulacral areas. Dorsally, next the apical disc (text-fig. 251, 

 p. 428), there are two ambulacral plates in an area. Both of these are primaries, therefore 

 crossing the half-area in which they lie. From this simple condition of primaries only, as a 

 localized stage, the number of columns increases, passing ventrally to the full number charac- 

 teristic of the species, and, in taking on this number, the ambulacrum passes rapidly through a 

 series of stages or phases in which there is the same number of columns that may be found as an 

 adult character in lower species of this genus. These dorsal localized stages in Lepidesthes 

 are directly comparable to the similar stages that I have shown in several species of Melon- 

 echinus (text-fig. 237, p. 231; pp. 366, 380). 



The interambulacra are narrow, half the width of the ambulacra, or less, and at the mid- 

 zone consist of four columns of plates which extend ventrally and dorsally as far as the areas 

 could be traced in the several specimens. In one area dorsally in one large specimen (Plate 69, 

 fig. 7; Plate 70, fig. 4) a fifth column originates 14 mm. from the top. This is the only case 

 of more than four columns seen in the species. The interambulacral plates are small, rounded 

 on the suture lines, and imbricate strongly aborally, also from the center laterally, and over the 

 ambulacra on the adradial sutures. In Lepidesthes wortheni (Plate 67, figs. 8, 11) and L. for- 

 mosa (Plate 68, figs. 5, 6) there is an odd number of columns of interambulacral plates, and in 

 these the middle column imbricates laterally in two directions, and the plates of this column 

 on the exterior are therefore wider than those of the lateral columns which imbricate laterally 

 only in one direction, to the left in the left half of the area and to the right in the right half of 

 the area. In Lepidesthes colletti with four columns of plates there is obviously no middle column. 

 In this case the plates of column 3 are usually wider, and imbricate laterally in two directions, 



