OCULAR AND GENITAL PLATES. 151 



adults of most Mesozoic and the young of all modern species of regular Echini. Passing to 

 Echinus esculentus, all exsert is still strongly the character, but one plate may be insert as an 

 infrequent progressive variant (p. 118). In Arbacia pundulata, a similar condition prevails, 

 but one plate insert is a more frequent variant and rarely two or three plates are insert. 

 Strongylocentrotus lividus has typically all oculars exsert, but one insert is a frequent and two 

 insert a rarer progressive variant (p. 126).^ Echinometra mathaei has a lower percentage of all 

 exsert and a consequent gain of progressive variants (p. 146). In Echinus angulosus the 

 progressive variants become more strongly marked, especially as compared with the northern 

 species of the genus shown (p. 119). 



In Salenocidaris varispina, instead of all exsert, one ocular insert is the species character, 

 and the feature of oculars all exsert passes into the phase of an arrested variant (p. 112). 

 Echinus magellanicus has typically one ocular insert; all oculars are exsert as an occasional 

 arrested variant, or I, V insert as an occasional progressive variant. In no case seen were 

 oculars I, V, IV insert, the space for which is indicated in the diagram at X, but in one speci- 

 men oculars I, V, IV, II are insert, as an extreme progressive variant (p. 119). Echinometra 

 lucunter from the West Indian fauna (p. 146) has one ocular insert as the species character, but 

 all exsert is a frequent arrested variant and two insert a more frequent progressive variant, with 

 rarely three plates insert. In the same species from Bermuda (p. 146) a different condition 

 prevails, as arrested variants are rarer and progressive variants are much commoner than in 

 the southern form. 



With Strongylocentrotus fragilis two oculars insert becomes the dominant character, with 

 all exsert or one insert as frequent arrested variants (p. 128). In Echinometra van brunti two 

 plates insert is strongly the dominant character with all oculars exsert or one insert as rela- 

 tively infrequent arrested variants; and three oculars insert as a somewhat rare progressive 

 variant (p. 147). Strongylocentrotus drobachiensis from York, Maine, has very strongly two 

 oculars insert, with but few arrested and progressive variants (p. 138). From here on, the 

 species shown have less of the bivium insert, few arrested variants with all exsert or one insert, 

 and progressively more with three or four plates insert. Toxopneustes variegatus shows a 

 distinct gain of progressive variants over the last species considered, and a consequent dropping 

 of the bivium insert character (p. 121). In Strongylocentrotus purpuratus there is a still further 

 reduction of the I, V insert and gain of the I, V, IV insert. A striking case is the Toxo- 

 pneustes atlanticus in which progressive variants are much in excess of what is found in the 

 closely allied T. variegatus (p. 122). 



With Tripneustes esculentus from the West Indian fauna (p. 124), three oculars insert 

 becomes the dominant character. In this species from this area two oculars insert is a very 



' In the diagram (text-fig. 176) the first 170 specimens only of Siroiigylocentrotus lividus are included. Recently, and 

 too late to alter the diagram, this number was increa.sed to 1,163 specimens. In this larger series the percentages (pp. 126, 

 162) are altered somewhat from that shown in the diagram and one specimen, 0.1% has oculars I, V, IV insert, a feature 

 not given in this diagram. 



