i6o Papers from the Marine Biological Laboratory at Tortugas. 



nance of V insert, 54 per cent, is first attained when the specimens are 25 

 to 30 mm. in diameter, about half grown, and the relative increase of V 

 insert progresses somewhat up to the largest series recorded. Having 

 oculars V, I insert is a progressive character for this species, and this feature 

 comes in late, progressively increasing in frequency up to the largest size 

 recorded. To have oculars V, I insert as a typical character is the feature of 

 Echinometra van hrunti A. Agassiz, from Lower California and South America, 

 and in this respect it is the most specialized species in the genus and family. 

 In the " Phylogeny of the Echini " much attention was given to ocular 

 plate arrangement and development of the same. The results for the Cent- 

 rechinoida are there briefly summarized in pages 92-94. As observations 

 on the Montego Bay series have materially extended the results In some 

 important particulars, a brief statement of the laws of ocular arrangement is 

 given here. In the " Phylogeny " the results of tabulating ocular plates in 

 young and adults of the order Centrechinoida are given for 48,541 specimens. 

 Those given above include in addition 2,878 specimens, or including both, 

 51,419 specimens. By the law of ocular development as worked out in 

 this order, the oculars when entering the periproct, do so in the sequence 

 I, VorV, I, IV, II, III (figs. 10-14), and it is seen by the following how closely 

 this rule is adhered to. Of the total 51,419 specimens of Centrechinoida 

 observed, oculars are all exsert in 6,763 specimens, all others having one or 

 more oculars insert. In 3,708 specimens, one ocular is insert, and of these 

 in 3,679 cases it is either ocular I (fig. 10) or ocular V, that is, 99.22 per cent 

 are correct by rule. There are only 29 exceptions.^ 1^35,951 specimens two 

 oculars are insert, and of these, in 35,611 cases, it is oculars I and V, or the 

 bivium (fig. 11), that is, 99.05 per cent are correct by rule. In all of the 

 340 exceptions^ one of the two plates insert is either I or V. In 3,820 speci- 

 mens, three oculars reach the periproct, and of these, in 3,426 cases, it is 

 oculars I, V, IV, the bivium and the left posterior plate of the trivium (fig, 12), 

 that is, 89.68 per cent are correct by rule. Of the 394 exceptions in 360 

 cases, the order is I, V, II insert (fig. 15), that is, the right posterior plate of 

 the trivium is insert instead of the left posterior as usual. These may 

 therefore be considered as right-handed specimens. Of the total 3,820 

 specimens with three oculars insert, 3,786, or 99.37 per cent, have either 

 oculars I, V, IV, or oculars I, V, II insert.^ In 619 specimens, four oculars 

 reach the periproct, and of these in 554 cases, it is oculars I, V, IV, II the 

 bivium and posterior pair of the trivium (fig, 13); that is, 89.50 per cent 

 are correct by rule.* In 558 cases all five oculars are insert (fig. 14). In the 



1 Of these exceptions, 18 have ocular IV alone Insert, three have ocular III aloue insert, and eight have 

 ocular II alone insert. 



' Of these exceptions, in loi cases, oculars I, IV are insert; in one case oculars I, III are insert; in s6 

 cases oculars I, II are insert, which is the species character in Gymnechinus robillardi (Loriol) and G. pulchellus 

 Mortensen; in 113 cases oculars V, IV are insert; in 4 cases oculars V, III are insert; and in 6s cases oculars 

 V, II are insert. 



' Of the other 34 exceptions to the rule where 3 oculars are insert, in 4 cases oculars I, V, III are insert; 

 in one case oculars I, IV, II are insert; in 11 casesoculars V, IV, III are insert; and in 18 cases oculars V, 

 IV, II are insert. 



< Of the 6s exceptions in 64 cases, the arrangement is oculars I. V, IV, III insert (fig. 16); this character 

 which is rare in the Centrechinoida as a whole, occurred principally in Centrechinus selosus (47 cases), but it 

 is a common character in the Cidaroida, the other 17 cases of I. V, IV, III insert are scattered through a number 

 of species of the Centrechinoida. One aberrant with four oculars insert has oculars I, V, II, III reaching the 

 periproct (fig. 17). This, which may be considered a right-handed equivalent of I, V, IV, III as faras experience 

 goes, is a unique variant in Echini. 



