OCULAR AND GENITAL PLATES. 123 



there are slightly more of I, V, IV, II insert as an extreme progressive variant and also more 

 aberrants. The largest series of 60 to 77 mm. in diameter, has about the same proportion of 

 arrested variants as in the two smaller sizes. Again, its progressive variants are practically of 

 the same frequency as in the next smaller series, demonstrating what I have found all through 

 Echini, that the incoming of oculars is a matter of individual development and not a matter of 

 age or size. Of aberrants in this species, 59 were found in the 2,643 specimens tabulated. 

 Of these all of the 31 specimens having V, II or V, IV, II insert, also have genitals 5, 1 fused, 

 which mechanically prevented ocular I from reaching the periproct, as in text-figs. 146 and 

 148, p. 134. The one specimen with oculars I, IV insert has genitals 4, 5 fused which 

 mechanically prevented ocular V from reaching the periproct, as in text-fig. 144, p. 134. 

 This accounts for a trifle over half of the 59 aberrant variants seen in the species (p. 164). 

 Of the remaining 27 aberrants, 25 are cases of I, V, II insert, as in text-fig. 145, which is a 

 right-handed equivalent of I, V, IV, and one specimen is a case of I, V, IV, III insert, which 

 is considered as reversionary to the character common in the Cidaroida, as in text-fig. 69, 

 p. 98. There is therefore only one variant, which has oculars I, V, III insert, that could in 

 any sense be considered as sporadic, a small number in so large a series. This species shows 

 almost exactly the same range of characters as Toxopneustes variegatus, but is markedly more 

 progressive than that species. The immature series as above described goes far toward 

 bridging the gap between the two, and a still younger would probably do so completely. In 

 this regard it may be noted that a small lot of 37 specimens of 30 to 35 mm. in diameter, 

 and not listed in the tables, has 92% with oculars I, V insert, no arrested variants, but 5.4% 

 with I, V, IV insert. This shows that at this age it is fully developed as regards the bivium, 

 but has relatively few of the progressive variants, less even than adult T. variegatus. 



The genus Tripneustes presents interesting characters with a considerable range. Of 

 Tripneustes depressus only 16 specimens were seen, but typically, 75%, oculars I, V are insert; 

 6% are arrested with I only insert; and 19% are progressive variants with I, V, IV insert. In 

 Tripneustes variegatus (72 specimens) oculars I, V are insert in 68 % as the species character. 

 Of arrested variants, 7 % have I only, and one specimen has V only insert. Progressive variants 

 are more common, 8% with I, V, IV and 11% with I, V, IV, II insert. Three specimens are 

 aberrant, two with I, II insert as in Gymnechinus pulchellus, and one with V, II insert as in 

 text-fig. 146, p. 134. 



Tripneustes esculentus is an interesting species on account of its range of characters in 

 ocular plates, both within a locality and in two widely separated areas (table, p. 161). In 193 

 specimens from Bermuda 61 % have oculars I, V insert as the typical character for that locality. 

 In 2% ocular I only is insert as an arrested variant (text-fig. 123). It is noteworthy that the 

 three specimens showing this character are large individuals, the one figured being the largest 

 seen in the species. Of progressive variants, 35 % have oculars I, V, IV insert, 2 % I, V, IV, II, 



