186 R. T. JACKSON — STUDIES OF PAL.EECHINOIDEA. 



figure 53). One of the interambulacral areas, C, has only 5 columns as 

 the greatest number, and this abortive introduction of a sixth in three 

 areas shows that 6 columns have not become a fixed specific character. 

 Another specimen has 5 columns in all 5 areas. One interambulacrum, 

 /, is too imperfect for details to be ascertained. Four columns of plates 

 are made out at the ventral border of the interambulacra as far as they 

 can be traced, but the specimen is imperfect at the lower portion of the 

 corona. New columns of plates are introduced by pentagonal plates with 

 adjacent heptagonal plates, as described in the several species of Melonites 

 and in Oligoporus coreyi and 0. dance. Dorsally the newly added plates 

 are more or less rhombic, as in Melonites and Oligoporus dame (plate 6,. 

 figure 34). Surface ornamentation of plates is like plate 6, figure 35. 



Part of the genital and ocular plates are preserved, and this is impor- 

 tant for comparison with the only other species of the genus Oligoporus 

 nohilis, Meek and Worthen, in which they have been observed. In Oligo- 

 porus nobilis, according to Meek and Worthen (31), 3 genital plates showed 

 5 pores, while 2 showed 4 ; the ocular plates are stated as imperforate. 

 In Oligoporus missouriensis (plate 9, figure 52) the form of the genitals 

 and oculars is the same as in Melonites (plate 3, figure 13). Two genitals 

 have 4 pores and one 3, instead of 5 and 4, as in Oligoporus nobilis. The 

 two oculars preserved are imperforate, as in that species and MelonileSy 

 and reach to the periproct. 



Oligoporus missouriensis differs from Oligoporus coreyi (plate 6, figure 28)» 

 w^hich has 6 columns of interambulacral plates, by its more massive pro- 

 portions; also by the stnaller relative size of ambulacral plates, more 

 abutting against an interaml)ulacral })late than in Oligoporus coreyi. It 

 differs from Oligoporus hlairi, Miller and Giirley (34), in the same features ; 

 also in being nearly circular instead of melon-like in form, as in that 

 species. It differs from all species of Oligoporus or Melonites seen or de- 

 scribed, in the nearl}^ circular form and in the fact that the interambu- 

 lacral plates at the junction with the ambulacra present a gently curving 

 outline much as in Archxocidaris (plate 8, figure 43), instead of an in-, 

 dented sutural line, as in Oligoporus dance (plate 6, figures 30 and 31); 

 also in the peculiar fan-like form of certain ambulacral plates, as de- 

 scribed. 



DESCRIPTION OF OLIGOPORUS COREYI. 



Oligoporus coreyi, Meek and Worthen (32), has never been figured, but 

 was described by its authors from a single specimen from the Keokuk 

 group of Crawfordsville, Indiana. A specimen in the collections of the 

 Museum of Comparative Zoology (catalogue number 3008), after care- 

 ful study, is referred to this species. This specimen (plate 6, figures 25, 



