152 ROBERT TRACY JACKSON ON ECHINI. 



common arrested variant in frequency closely approaching the dominant character, and four 

 oculars insert is a common progressive variant; rarely all oculars are insert. Arbacia nigra has 

 strongly three oculars insert with no progressive variants, but occasional arrested variants of 

 all exsert, and frequent arrested variants with one or two oculars insert (p. 116). This species 

 covers the same range of characters as shown in Arbacia punctulata, but in very different pro- 

 portions. Centrechimis setosus has strongly three oculars insert as the species feature and is 

 almost the only species in the order in which the variation covers six characters. All exsert 

 or one or two oculars insert occur as rare arrested variants, and four oculars or all insert are 

 frequent progressive variants (p. 108). Of Centrostephanus rodgersi there are few observations, 

 but as far as they may be trusted, it is the strongest three oculars insert of any species known, 

 with frequent progressive variants having four or all oculars insert (p. 110). Eiicidaris tribu- 

 loides has strongly three oculars insert as a dominant character (p. 97). Its range of varia- 

 tion covers practically all the characters known in regular Echini from the Lower Carboniferous 

 up. The only exceptions to this statement are cases of Echini that typically have an aberrant 

 ocular arrangement, as Gymnechinus pulchellus (p. 120) and Strongylocentrotus gibbosus (p. 145). 

 The arrested variants of Eucidaris tribuloides are few, with all oculars exsert, 'or one or two 

 insert. Progressive variants are common with four or much oftener five oculars insert. In 

 no sea-urchin studied is four oculars insert a typical species character, though this is said 

 (Cotteau, 1875-'80) to be the case in Acrosalenia pseudodecorata Cotteau. 



With Phyllacanthus annulifera all oculars insert is the character with three or four oculars 

 insert as frequent arrested variants (p. 102). In Phyllacanthus baculosa all oculars are insert as 

 a strong species feature (p. 102). The only possible variants on this line of development are 

 arrested, and such occur with three or four plates insert as rare arrested variants. In Aspido- 

 diadema nicobaricum all oculars are strongly insert and no variation is known (p. 104). Those 

 cases given in the diagram are selected, but almost any of the species shown in the tables of 

 ocular plate arrangement could be intercalated in the diagram and there find an harmonious 

 place in the scheme of differential structure. 



The accompanying tables show the characters of ocular plates in the species studied. 

 The table for the Cidaroida is on pp. 100, 101 and for Strongylocentrotus drobachiensis on pp. 142, 

 143. In tabulating characters the only specimens omitted were those that had no locality, 

 where this was an important item; also 71 that had three, four, or six areas, and some eight speci- 

 mens that were so distorted that they could not properly be included. The general relation of 

 percentages is considered on p. 93. All aberrant variations in the Centrechinoida are given in 

 the last table (p. 164). As the relation of ocular plates to the genitals and periproct plaj's 

 such a distinct and definite part in geological sequence, in ontogeny, in variation, and in 

 geographical distribution, it seems that they may be accepted as a feature of importance in 

 the study of Echini. It seems also that they make a reasonable basis where differences occur 

 on which to arrange the systematic sequence of species within the genus. 



