16 PROF. P. M. Duncan's revision of the 



The obliquity of the edges of the interradial plates is slight, 

 and its direction and amount are insufficient for imbrication; 

 the ambulacral plates, when thin, have some imbrication adorally 

 and laterally. 



Fossil. Carboniferous : England, Europe ; N. America. 



Genus Oligoporits, MeeJc Sf Wbrfhen, 1860, Proc. Acad. Nat. 

 Sci. PMlad. vol. xii. p. 474; 1866, Pal. Illinois, vol. ii. 

 p. 247. 



Test with the same general shape as Melonites, and the apical 

 system also. 



Ambulacra with four vertical rows of plates, each plate per- 

 forated by a pair of pores, some demi-plates amongst the pri- 

 maries. 



Interradiu large, convex, with from five to nine vertical rows 

 of plates at the ambitus, diminishing in number towards the 

 poles. 



Fossil. Lower Carboniferous : N. America. 



Genus Lepidesthes, Ifeek Sf Worthen, Pal. Illinois, vol. iii. 

 p. 522. 



Test ellipsoidal ? 



Ambulacra very broad, consisting of ten vertical series of plates 

 at the ambitus ; the plates overlap adorally, and they are broad, 

 small, low, and each has a pair of pores. 



Interradia comparatively narrow, with five or six vertical rows 

 of plates, which overlap aborally and at the sides. Tubercles 

 very small, equal. 



Fossil. Carboniferous : N. America. 



The next genus requires careful consideration, for it was 

 founded partly by Worthen, whose name is so familiar to 

 students of the Palseechinoidea, and yet contains characters 

 which appear to be due to the same cause which led some excel- 

 lent observers into error with regard to the nature of the imbri- 

 cation of the Echinothuridge. 



It appears that it is quite possible that the distinguished 

 American palaeontologist may have seen the plates of his type 

 from within, and, if so, it accounts for the character which he has 

 given the type, of the plates overlapping in a direction contrary 

 to all other Palseechinoidea. Worthen and Miller, after noticing 



