94 ROBERT TRACY JACKSON ON ECHINI. 



This last is the dominant character of Strongylocentrotus gibbosus (text-fig. 156, p. 145), in 

 which species a commensal crab profoundly modifies the apical disc. 



In 387 specimens, four oculars reach the periproct, and of these in 340, which is 87.86 % 

 of the cases, the order is I, V, IV, II, the bivium and posterior pair of the trivium (text-fig. 

 126, p. 124). Of the 47 exceptions, the order in all is I, V, IV, III insert (text-fig. 149), and 

 this order is common as a character in the Cidaroida (text-fig. 69, p. 98). It is interesting to 

 note that 32 of the 47 cases of this aberration occur in Centrechinus, which genus is relatively 

 near the base of its order, and therefore nearer to the Cidaroida. When, therefore, four oculars 

 are insert in the Centrechinoida in all, or 100%, the arrangement is that characteristic of the 

 order, or is reversionary to the character frequent in the Cidaroida. In 518 specimens all the 

 oculars are insert. 



Of the total 48,541 specimens of Centrechinoida observed and tabulated, when oculars 

 become insert, they come in in the order of sequence I, V, or V, I, IV, II, III in 98.55%, and 

 they are out of order, or aberrant for the Centrechinoida in 704 cases, or 1.45%. 



Of the above considered aberrants as there stated, certain combinations of ocular plate 

 arrangement can be definitely accounted for, the I, V, II insert (300 specimens) as right-handed 

 individuals (text-fig. 145, p. 134); the I, IV (97 specimens) are incomplete I, V, IV by the 

 usual exclusion of ocular V, due to the fusion of genitals 4, 5 (text-fig. 144, p. 134) ; the V, II 

 (63 specimens) text-fig. 146, as incomplete I, V, II, due to the usual fusion of genitals 5, 1, 

 excluding ocular I; the V, IV, II (17 specimens), text-figs. 148, p. 134 and 196, p. 169, may be 

 considered as incomplete I, V, IV, II, due to the exclusion of ocular I by the fusion of genitals 

 5, 1 ; the I, IV, II (1 specimen), text-fig. 147, (p. 134) is similarly accounted for by the exclusion 

 of ocular V. Finally, the I, V, IV, III (47 specimens), text-fig. 149, (p. 134) is accounted for 

 as a reversion to the character typical of some of the Cidaroida. The sum of these definitely 

 accounted for variants, which is 525, subtracted from the total number of aberrant variants, 

 which is 704, leaves only 179, or 0.37 %, which can be considered as distinct departures from 

 the rule of ocular plate arrangement. Of these last, 56 specimens have oculars I, II insert 

 (text-fig. 141, p. 134), which is the species character of Gymnechinus pulchellus and robillardi 

 (text-figs. 177-179, p. 165), and 82 specimens have oculars V, IV insert (text-fig. 142, p. 134), 

 which is a left-handed equivalent of the I, II. insert. It is evident that even aberrants follow a 

 quite definite arrangement and sporadic variants are extremely rare, only 41 cases as a whole 

 in the 48,541 specimens of the order examined. Of these last, which are considered sporadic 

 variants, ten specimens have ocular IV alone insert, three have III alone, and eight have 

 ocular II alone insert; one specimen has oculars I, III insert, and four have V, III insert; 

 four specimens have oculars I, V, III insert, and eleven have oculars V, IV, III insert. The 

 detailed distribution of aberrants in families, genera, and species of the Centrechinoida can 

 be seen in the table of aberrant variants, p. 164. 



