. Carnegie Institution of Washington publication. Studies of Jamaica Echini. 145 insert; 3 per cent are aberrant variants, all five specimens having oculars I, V, IV, III insert. Considering the ocular-plate arrangement in Centrechinus from Montego Bay as a whole, it is seen, as shown clearly in table 2 (page 156), that from all character of the young, there is a steady progressive series with growth in the development or traveling in of ocular plates. Centre- chinus is a member of the most primitive suborder, the Aulodonta, of the Centrechinoida, and as such is worthy of special at
. Carnegie Institution of Washington publication. Studies of Jamaica Echini. 145 insert; 3 per cent are aberrant variants, all five specimens having oculars I, V, IV, III insert. Considering the ocular-plate arrangement in Centrechinus from Montego Bay as a whole, it is seen, as shown clearly in table 2 (page 156), that from all character of the young, there is a steady progressive series with growth in the development or traveling in of ocular plates. Centre- chinus is a member of the most primitive suborder, the Aulodonta, of the Centrechinoida, and as such is worthy of special attention, for, as shown by Professor Hyatt, primitive types in a group have a slow development as com- pared with specialized types, and in character approach nearest to the next lower series of their own phylum. The development of Centrechinus amply bears out this important truth. Of the total 1,401 specimens of Centrechinus setosus listed from Mon- tego Bay, the aberrants are 59 in number, of which 8 have ocular IV only insert; 3 have oculars I, IV insert; 32 have oculars V, IV insert; i has oculars I, V, II insert, and 15 have oculars I, V, IV, III insert. It is note- worthy that in the mature series (50 to 80 mm. diameter) the only aber- rants that occurred were cases of oculars I, V, IV, III insert, thus indicating that the other aberrants seen in immature individuals may be considered as irregularities in the order of sequence of the coming in of ocular plates, rather than as aberrants that would have retained the given character if they had lived to grow up. In specimens of Centrechinus up to 14 mm. in diameter (48 individuals), all the genital plates are imperforate (fig. i), whereas in those mm. and all larger with very few exceptions, all genital plates have the genital pores.^ In comparison with this, the specialized Strongylocentrotus drohachiensis attains its genital pores when between 5 and 10 mm. in diameter (Phylogeny of the Echini, pp. 131, 170). Centrechin
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