CLASSIFICATION OF PALEOZOIC ECHINI. 241 



ambulacral plates and probably 9 columns of interambulacrals. As this 

 species, M. etheridgii, has up to 14 columns of plates, it is by our accepted 

 standard the most specialized species of the genus. 



Lepidesthidse. — This family is characterized by 6 or more columns of 

 ambulacral plates, which associate it systematically near to the Melo- 

 nitidse ; it differs from the Melonitidse in having imbricated plates and 

 pores in the center of ambulacral plates. 



Lepidesthes wortheni (plate 9, figure 53), the simplest species, has 

 4 columns of ambulacral plates ventrally, as in Melonites ; they increase 

 to 7 or 8 columns at the ambitus. In the interambulacra there are 4 

 columns of plates ventrally, decreasing to 3 at the ambitus. In another 

 specimen there are 3 columns throughout the length of these areas (page 

 208). Another species, L. fonnosus, Miller, has 8 ambulacral and 5 inter- 

 ambulacral columns. The next species, L. spectabills,^ has 10 ambulacral 

 columns, as has also the third species, Z. coreyi, M. and AV. The most 

 specialized species, L. colletti, White, in regard to ambulacral develop- 

 ment is the most specialized of all Paleozoic Echini, having many more 

 columns than any other known species. 



Pholidocidaris (see plate 9, figure 54) is a very distinct genus on account 

 of the peculiarities of its ambulacral plates and the irregularity and strong 

 imbrication of both ambulacral and interambulacral plates. Better ma- 

 terial in this genus is much to be desired. The two species, P. irregularis, 

 M. and W. and meeJci, .lackson, are about on a par as regards differentia- 

 tion of structure. 



Archseocidaridge. — Archseocidaris (plate 8, figures 43-46) as a genus is not 

 characterized by a great development of interambulacral plates and has 

 only 2 columns of ambulacrals. One striking feature is the extensive 

 resorption of the ventral border of the corona by the encroachment of the 

 peristome, as shown by A. wortheni. Many species have been described 

 from dissociated spines and plates, but obviously they are wanting in 

 those characters which would enable one to include them in a classifica- 

 tion based on a required knowledge of a more or less complete corona. 

 Archseocidaris wortheni, Hall, has 4 columns of interambulacral plates. 

 A. drydenensis,Ya,n\ixem. has 7 columns of interambulacral plates, and is 

 therefore placed above as being more specialized. 



Lepidocidaris (see plate 7, figure 41) in the sum of its characters is placed 

 close to Archaeoeidaris, but it has more than 2 columns of ambulacral 

 plates. These additional plates were apparently formed by a drawing- 

 out process, as in Oligoporus (compare plate 7, figure 41 and plate 6, 

 figure 25). 



Lepidocentrid^e. — This family is systematically nearest to the Archse- 



*This species was described as Hyboechinus spectnblUs. See statement, page 207. 



