PLATE ARRANGEMENT IN OLIGOPORUS COREYI. 189 



are from 4 to 9 columns of plates, varying with the species. It is shown 

 in preceding pages that the young of Meloniies, as indicated hy the ventral 

 portion of the corona, in two species is like adult Oligoporus in having 4 

 columns of ambulacral plates ; therefore the ventral portion of Oligoporus 

 assumes a special interest, as by it the relations of the two types may 

 be carried back one, two, or even three steps further. 



The ventral termination or younger portion of Oligoporus coreyi is shown 

 in plate 6, figure 25. First taking up the ambulacral areas, it is seen that 

 the ambulacrum terminates ventrally in two plates, a, b. The plates do 

 not run directly across the half area as in Cidaris (plate 8, figure 48), but 

 slightly overlap one another, as seen more markedly in the ambulacrum 

 of Rhoechinus gracilis and elegans (plate 7, figures 37 and 40). Passing 

 dorsally, the overlapping of the plates of the ambulacrum in Oligoporus^ 

 plate 6, figure 25, progressively decreases, passing through a stage which 

 may be compared with the characteristic condition of Palseechinus (plate 

 7, figure 39) until in the tenth row on the right side of the area it is seen 

 that there are two plates, a, a\ in one-half of the ambulacrum, which 

 interlock at their median points of contact almost exactly as they do in 

 the adult of Oligoporus danse (plate 6, figure 30). 



It is observed that the pores of the ambulacral plates in Oligoporus 

 coreyi are in the center of the half-areas instead of being close to the 

 interambulacral area, as seen in 0. danse (plate 6, figure 30). This is 

 because the specimen is viewed from the inner or proximal side of the 

 test, and the pores in passing through the substance of the plate are in- 

 clined, so that while they are toward the interambulacral area on the 

 outer or distal side of the plate on the inner side they appear much nearer 

 the center of each half-ambulacral area (see page 184). 



We have in this development of the ambulacrum of Oligoporus the 

 highly interesting fact that ventrally the plates of that area are in two 

 instead of four rows, and are in addition closely like the plates of 

 Rhoechinus ; during growth and before attaining the mature condition 

 they pass through a stage like that characteristic of Palseechinus ; also 

 the final important fact that the number and arrangement of ambulacral 

 plates of adult Oligoporus is like that of the ventral or younger portion of 

 Melonites (plate 2, figure 2). As the ambulacral areas are the real feature 

 of generic distinction between Rhoechinus^ Palseechinus^ Oligoporus and 

 Melonites, it is with great satisfaction that I am able to state that the four 

 genera may be most intimately and serially connected by a study of the 

 growth of the ambulacrum. The four genera represent a distinct phjdum, 

 which is progressively advancing in the line of increase of columns of 

 ambulacral plates. Etheridge (11) has pointed out already from a con- 

 sideration of the adult that Oligoporus seems to be intermediate between 



