SYSTEMATIC CLASSIFICATION OF ECHINI. 233 



The characters of the genus Perischodomus M'Coy (Plate 62, figs. 6, 7; Plate 64, figs. 2-8) 

 are taken from P. biserialis M'Coy, the only satisfactorily known species. The ambulacra 

 have two columns of plates in each area at the mid-zone, they are alternately wedge-shaped, 

 or a plate may be quite shut out from interambulacral contact; pore-pairs are more or less 

 biserial, but dorsally all plates are similar primaries with pore-pairs uniserial. Interambu- 

 lacra have five columns of rounded strongly imbricate plates in each area; the plates of the 

 adradial columns bear each an eccentric perforate pi'imary tubercle. The primordial inter- 

 ambulacral plates are in the basicoronal row, there being no coronal resorption. Genital 

 plates have numerous pores. This genus has several characters like the Lepidocentridae, in 

 which family I previously placed it, but the sum of the evidence seems to place it in this family. 

 One additional species, P. illinoisensis, is known incompletely. 



The genus Perischocidaris Neumayr (Plate 65, figs. 1, 2; Plate 67, figs. 1-3) is known only 

 from one species, P. harteiana (Baily). It has a high rounded test with six columns of plates in 

 depressed furrows in each ambulacral area. The interambulacra have five columns of rounded 

 hexagonal plates which indicate moderate imbrication. The actual plates are not preserved 

 as the only known specimens are two external sandstone molds. Part of the adradial inter- 

 ambulacral plates bear an eccentric perforate primary tubercle and all interambulacral plates 

 bear secondary tubercles. Dorsally three of the interambulacral columns drop out, so that 

 in each area only two columns exist next the apical disc, a degree of dropping out seen in 

 no other Palaeozoic species. The ocular plates are all insert and genitals have many pores. 

 The ventral half of the test is unknown. 



Proterocidaris Koninck (Plate 65, fig. 3; Plate 67, figs. 4-7) contains only one species, 

 P. giganteus Koninck. I have not seen specimens, but observations were made from Fraipont's 

 excellent photographic figures, which he refers to Oligoporus soreili Fraipont. It is certainly 

 not Oligoporus, and is referable to de Koninck's species. The test is very large, apparently 

 nearly or quite spherical. There are four columns of plates in each ambulacral area and from 

 eleven to thirteen columns of plates in each interambulacral area, varying in areas of the same 

 specimen. This is the only species in the family with many interambulacral columns. The 

 plates are strongly imbricate and bear rather small primary with numerous secondary spines. 



Lepidesthes Meek and Worthen, is a most interesting genus; the species are mostly small 

 and elliptical or spherical. The ambulacra are wide and in each area consist of from eight to 

 sixteen columns of very regular rhombic or more or less hexagonal plates, and the interambulacra, 

 which are relatively narrow, of from three to seven columns of plates in each area. Plates are 

 thin, strongly imbricating, bearing secondary spines and tubercles only. The primordial inter- 

 ambulacral plates are in the basicoronal row, and the peristome is covered with ambulacral 

 plates only. Ocular plates are insert, or in one species exsert, and genitals have few pores. 

 The periproct is covered with many moderately thick plates. This is one of the most extreme 



