242 R. T. JACKSON — STUDIES OF PAL^ECHINOIDEA. 



ocidaridse, but has essential differences in the strongly imbricated char- 

 acter and also unusual form of interambulacral plates. Another differ- 

 ence of significance is the small i^eristome and the correlated retention 

 of the initial interambulacral plate in the adult (Lepidechinus, plate 7, 

 figure 42). 



Lepidocentrus (see figure 2, page 223) is considered the central type be~ 

 cause its interambulacral plates, though specialized in form, are nearest 

 the usual form of plates seen in normal types, esj^tecially the 3^oung, re- 

 cently added plates at the dorsal portion of the corona (see Melonites, 

 plate 3, figure 13). L. rhenanus, Beyrich is considered the least special- 

 ized species, because its interambulacral plates are nearly hexagonal and 

 therefore show less modification from normal types than the other species 

 of the genus. It is also simpler in having fewer columns of interambu- 

 lacral plates than the next species. L. muUeri, Schultze has 11 columns 

 of interambulacral plates, and its plates are quite near to rhombic in form 

 throughout the entire test. 



Lepldechiiiii.i (see plate 7, figure 42) is a genus quite distinct from 

 Lepidocentrus, but only one species, L. rartspimis, Hall, is at all well 

 known. This species has strongly imbricated plates. The initial inter- 

 ambulacral plate is retained in the adult and it has a highly accelerated 

 development. In the table the species L. imhricatus, Hall is put first be- 

 cause as described it has fewer columns of interambulacral plates than 

 L. rarispinus. 



Perlschodomus (see Keeping (22)) differs essentially in the form of its 

 plates from other genera of the family, the plates being more nearly 

 hexagonal than in other genera. The species from this country, P. 

 ilUnoisensis, is not well known. 



Order III, Cystocidaroida. — This order is represented by the genus Echino- 

 cysliteA, Wyv. Thomson (39). It is unsatisfactor}' to locate systematically 

 on account of conflicting evidence. In the table, therefore, the order is 

 given, but without descriptive detail. This is done because, while not 

 wishing to omit altogether so important a group as a whole order of Paleo- 

 zoic Echini, I was unwilling to introduce confusion by giving details of 

 structure which were apparentl}'' out of place in the systematic scheme. 

 The details of structure are given at this place. 



(Eu.) E. pnmum, Wyv. Thomson, 4 A., 8 I. 

 (Eu.) E. Kva, Wyv. Thomson, 4 A., 6 I. 



EcHiNOCYSTiTES, Wyv. Thomson, 4 A., 6-8 I. 

 I Up]~>er Silurian. 



Order III. CYSTOCIDAROIDA, 4 A., numerous I. Peristome small, 



periproct eccentric. Plates imbricated. 



