COMPARISON OF OLIGOPORUS WITH MELONITES. 199 



The above described specimens of the genus Oligoporus, while not nu- 

 merous in individuals observed, are sufficient to make a fair relative com- 

 parison with the genus Melonites. The relations of the ambulacra have 

 already been described in the two genera under the consideration of Oli- 

 goporus coreyi (page 191). The ambulacra of Oligoporus begin ventrally as 2 

 columns of plates, and proceeding dorsally these 2 become 4 by the pull- 

 ing apart of one row on each side to make 2. Melonites has 4 columns of 

 ambulacra at the ventral border, like adult Oligoporus, and these increase 

 by the addition of new columns during progressive growth of the in- 

 dividual to 10 columns in Melonites multiporus, Norwood and Owen ; 12 

 in M. giganteus, Jackson and 12, or 14, in M. etheridgii, Keeping (21"). 

 There are 4 columns of ambulacra in Oligoporus in all the species. In 

 the feature of ambulacral areas Oligoporus is therefore distinctly more 

 primitive in the scale of organization than Melonites. In the interambu- 

 lacra there is a progressive development in both genera from two col- 

 umns represented by two plates at the ventral border to many columns 

 in the adult. Therefore those species with few columns are obviously 

 more primitive in organization in this respect than species with many 

 columns. The number of columns in the interambulacra of Oligoporus 

 varies from 4, as described in 0. bellulus, Miller and Gurle}^ (34), and 5 

 in 0. nohilis, M. and W., to 6 in 0. blairi, Miller and Gurley (34), and 

 coreyi, M. and W., and finally 9 in 0. danse, M. and W. In Melonites the 

 columns of interambulacra vary in species from 4 in M. dispar, (Fischer) 

 (12) to 5 in M. crassus, Hambach (18), to 6 in M. indianensis, Miller and 

 Gurley (34) ; 7 in M. septenarius, Whitfield ; 7 to 8 or 9 in il£ multiporus, 

 Norwood and Owen, and finally 11 in M. giganteus, Jackson. Species of 

 Oligoporus, then, have from 4 to 9 columns of interambulacra, and species 

 of Melonites have from 4 to 11 columns. Therefore the sum of the species 

 of Oligoporus, as compared with the sum of the species of Melonites, shows 

 that in regard to the interambulacra, as well as the ambulacra, Oligoporus 

 is more primitive as a genus than Melonites. 



Both Oligoporus and Melonites in their development progressively in- 

 crease from 2 and doubtless earlier from 1 interambulacral plate in the 

 young to many columns in the adult, which indicates a common ancestor 

 with one and intermediate common ancestors with few columns in the 

 adult. Accepting this, Melonites should be considered a further remove 

 from the primitive than Oligoporus, because its species acquire more 

 columns of plates. Species with few columns of plates may be consid- 

 ered more primitive, other things being equal, because they represent in 

 the adult condition, stages passed through in their development by those 

 species having a larger number of columns in the adult. 



It seems that Melonites may be considered the extreme genus of the 



