444 ROBERT TRACY JACKSON ON ECHINI. 



one or two plates in an ambulacral area, from which simple condition as a localized stage, the 

 number of columns of plates increases passing ventrally until the full number is attained at or 

 near the ambitus. The ambulacral plates are low, wide rhombs (Plate 76, figs. 2, 3), and imbri- 

 cate strongly adorally and laterally bevel under the adradials. The pore-pairs are situated in 

 peripodia, and in each plate lie a little nearer the next adjacent interambulacrum than the 

 middle of the plate. Each ambulacral plate bears a small perforate primary tubercle with a 

 scrobicule and also a number of small secondary tubercles. The larger tubercles bear small 

 primary spines which are terete, swollen at the base, and longitudinally finely striate, measur- 

 ing about 3 mm. in length. On account of their small size it might be doubted whether these 

 should be called primary spines, but I do so because they are associated with the larger, 

 minutely perforate tubercles which bear a scrobicule. The secondary spines are similar but 

 smaller, and measure up to about 1 mm. in length. 



Besides spines, a number of pedicellariae were found in these choice specimens (Plate 76, 

 figs. 8, 9). The pedicellariae are tridentate, similar to the character found in Recent Echini, 

 and measure about 0.5 mm. in length. They are the first pedicellariae found in the Palaeozoic. 

 It is interesting to find that these minute structures of the common typical form, as seen in 

 modern Echini, already existed in these ancient times. Groom (1887) described pedicellariae 

 in the Jurassic Pelanechinus corallinus and these with the pedicellariae of Meekechinus are the 

 only ones known to me so far found as fossils (p. 61). 



The interambulacra are relatively and actually very narrow as above stated, and as far 

 as preserved, consist of three columns of plates throughout each area. Except Bothriocidaris 

 which has one column, and Miocidaris which has two columns in each area, all Palaeozoic 

 Echini have three or more columns of plates in each interambulacral area. Only rarely are 

 three columns characteristic of the adult, but a three-column stage is typically represented by 

 one row of plates ventrally in species that attain a higher number of columns (text-figs. 25, 

 30, p. 70). Three Palaeozoic species have three columns as an adult character: the present 

 species, Melonechinus obovatus sp. nov. (p. 374; Plate 53, figs. 9, 10), and Lepidesthes wortheni 

 Jackson (p. 416; Plate 66, figs. 1-3; Plate 67, figs. 8, 9). This last species, however, has four 

 columns ventrally as a stage in early development. The interambulacral plates of Meekechinus 

 elegans are small, rounded on the suture line, about as high as two ambulacral plates, and 

 imbricate strongly aborally and from the center laterally and over the ambulacra on the 

 adradial suture (Plate 76, figs. 4, 5). Each plate typically bears a small, central, perforate 

 primary tubercle with a scrobicule and many small secondary tubercles. On the other hand, 

 in Pholidocidaris the primary tubercles are eccentric, and at least dorsally are wanting on the 

 plates of median columns. The spines of interambulacral plates are small primaries and 

 secondaries, similar to those described in the ambulacra. The plates of the ventral area are 

 known only from a fragment of the test associated with the lantern (Plate 75, fig. 8). Here, 



