234 ROBERT TRACY JACKSON ON ECHINI. 



genera of Echini for ambulacral differentiation. The best known species are Lepidesthes for- 

 mosa (Plate 66, figs. 4-7; Plate 68, figs. 3-14) and L. colktti (text-figs. 14, p. 54; 32-38, p. 75; 

 251; Plates 69, 70; Plate 71, fig. 1). 



Pholidocidaris Meek and Worthen is one of the most specialized in certain respects of 

 Palaeozoic genera. There are from four to six columns of plates in each ambulacral area, but 

 the plates are very large ventrally and small dorsally. A peculiarity is that the pores are 

 situated in the middle of the plates, a primitive character. The interambulacra as far as 

 known have five or six columns of plates in an area and the plates are rounded polygonal. The 

 adambulacral plates dorsally are very much larger than the intermediate plates, one equaling 

 in height the distance of two or three of the adjacent interambulacral plates. This is a character 

 known in no other Echini. In addition the adambulacral plates bear each one eccentric primary 

 spine and perforate tubercle, with secondary tubercles. The intermediate plates dorsally 

 bear secondary tubercles only. The primordial interambulacral plates are in the basicoronal 

 row, and the imbrication of coronal plates is extreme. The genital plates have many pores. 

 No species is known perfectly, but the best known is the genotype, Pholidocidaris irregularis 

 (Plate 73, figs. 3-7; Plate 74, figs. 1-7; Plate 75, figs. 1-5). 



Meekechinus gen. nov. (Plate 75, figs. 6-8; Plate 76) is in some respects the most specialized 

 of known Echini, and is based on the single species M . elegans sp. nov. There are twenty 

 columns of low rhombic plates at the mid-zone in each ambulacral area, the highest number 

 known in any sea-urchin. With this great expanse of ambulacrum there are only three columns 

 of small plates in each interambulacral area. All plates are strongly imbricate, all interam- 

 bulacral plates bear a small primary spine and tubercle, with secondary tubercles, with the same 

 on the ambulacral plates. Ocular plates are apparently all exsert, and genitals are large, the 

 madreporite being exceptionally large and with distinct madreporic pores, the clearest seen in 

 any Palaeozoic species. Pedicellariae were found in this remarkable species, and are the only 

 ones yet known from the Palaeozoic. The lantern has the typical Palaeozoic characters except 

 that the teeth are distally denticulate, a unique character for Echini. This species is the 

 highest geologically and the most specialized structurally of any known Palaeozoic echinoid. 



If this classification is adopted the names Palaeechinoida and Euechinoida will naturally 

 lapse as the major divisions of the Echini are ordinal only as shown in the table, p. 209. 



