364 ROBERT TRACY JACKSON ON ECHINI. 



Melonechinus occurs in the Lower Carboniferous of America, Great Britain, and Russia. 

 The type species is Melonechinus multiporus (Norwood and Owen). 



Key to flu: Species of Melonechinus. 



Six columns of ambulacra! and four columns of interambulacral plates in each area 



M. (lispar (Fischer von Waldheim), p. 365. 



Six columns of ambulacral plates, and at the initl-zone fi\e columns of interambulacral plates, a sixth 

 column coming in dorsally in each area M. parvus (Hambach), p. 365. 



Six columns of ambulacral plates, and six columns of interambulacral plates at or just abo\e the mid-zone, 

 with a strong tendency to adding a seventh column dorsally in each area . M. springeri sp. nov., p. 366. 



Eight columns of ambulacral and six columns of interambulacral plates in each area, melon-like ribs low, 

 gently rounded . M. crassus (Hambach), p. 367. 



Eight columns of ambulacral anrl six columns of interambulacral plates; a seventh interambulcral 

 column may appear dorsally, or exceptionally, e\en an eighth column in each area; melon-like ribs high, 

 sharply elevated M. indiancns-is (Miller and Gurley), p. 360. 



Eight columns of ambulacral plates and seven (or in one area of the type, six) columns of interamijulacral 

 plates in each area. Melon-like ribs strongly and sharply elevated . . M. sfewarfii (Safford), p. 371. 



Eight columns of ambulacral and se\en columns of interambulacral plates at the mid-zone in each area; 

 test small M. scpfenarius (.Jackson), p. 373. 



Ten columns of ambulacral and three columns of interambulacral plates in each area; test obovate 



31. obovafus sp. nov., p. 373. 



Ten columns of ambulacral and eight colunms of interaml)ulacral plates in each area; high, steeply 

 rounded, melon-like ribs in ambulacral and interambulacral areas . . . M. liratus sp. nov., p. 374. 



Ten columns of ambulacral and dorsally eight or nine columns of interambulacral plates (rarely seven 

 columns only in an area) in each area; low, rounded, melon-like ribs in ambulacral and interambulacral 

 areas M. jnuUiporus (Norwood and Owen), p. 375. 



Twehe columns of ambulacral and six columns of interambulacral plates in each area; melon-like 

 ribs wanting M keepingi sp. nov., p. 384. 



Twelve columns of ambulacral and seven columns (possibly more) of interambulacral plates in each area; 

 melon-like ribs wanting M. etheridgii (Keeping), p. 385. 



Twelve columns of ambulacral and nine columns of interambulacral plates in eacii area; melon-like 

 ribs strongly developed M. raiulerhilii sp. no\-., p. 388. 



Twelve columns of ambulacral and cle\ en colunms of interambulacral plates in each area; melon-like 

 ribs strongly developed .1/. giganteus (Jackson), p. 389. 



' Melonites { = Melonechinus) youngi, of which no description has been published, is considered under Nomina Nuda, 

 p. 457. 



