150 ROBERT TRACY JACKSON ON ECHINI. 



metra lucunter (text-fig. 159, p. 147). Two plates may be insert, when it is typically the 

 bivium, as in Strongylocentrotus franciscanus (text-fig. 166). Three plates may be insert, when 

 it is the bivium and left posterior plate of the trivium, as in Arbacia nigra (text-fig. 167). 

 When four plates are insert, it is typically the bivium and posterior pair of the trivium, as in 

 Acrosalenia pseudodecorata (text-fig. 168). I have shown this character in many cases as a 

 variant but have not seen a species in which it is the character, so have copied the figure 

 from Cotteau (1875-'80), as he says that four plates are typically insert in that species. All 

 ocular plates may be insert, as in Phyllacanthus baculosa (text-fig. 169), when we have a 

 character which is a close approach to that dominant in the Upper Palaeozoic (text-fig. 163). 

 The next differential character to appear is for interspaces to develop between the ocular and 

 genital plates, so that the periproct comes in contact with the interambulacra, as in Phormo- 

 soma placenta (text-fig. 170). 



Turning to the irregular Echini, we find in part a parallel series to that seen in the regular 

 Echini. As the periproct is eccentric, of course oculars cannot reach that structure, but if 

 oculars reach the center line, they may be considered insert, or if excluded from the center line 

 by junction of the genital plates, they may be regarded as exsert. Oculars may be all exsert, 

 as in Holectypus (text-fig. 171; compare text-fig. 164). The bivium alone may be insert, as 

 in Cassidulus (text-fig. 172; compare text-fig. 166). In some species, as Toxobrissus (text-fig. 

 173), the plates of the bivium may be separated by the posterior extension of the madreporite, 

 but as this plate radially speaking is out of place, the oculars I, V may still be considered as 

 morphologically insert. Or it could be looked at from another point of view and ocular V alone 

 regarded as insert (as is common in the Arbaciidae), the other four oculars being shut out 

 from the center by the contact of the genitals. Three ocular plates may be insert, when it is 

 the bivium and left posterior plate of the trivium, as in Micraster (text-fig. 174; compare text- 

 fig. 167). Or, finally, four ocular plates may be insert, when it is the bivium and posterior 

 pair of the trivium, as in Ananchytes (text-fig. 175; compare text-fig. 168). 



The typical character of arrangement of ocular plates and the range of variation are ex- 

 pressed diagrammatically by block lines in text-fig. 176. Each block represents the frequency 

 of the character in the species, as given in the tables (pp. 100, 101, 143, 154-163). An excep- 

 tion to this statement is in the cases of Strongylocentrotus lividus (p. 126) and Phyllacanthus 

 annulifera (p. 102) in which additional specimens, tabulated after this diagram was drawn, 

 somewhat altered the facts and percentages, but it was not deemed of sufficient importance to 

 omit the observations, and there was no opportunity to alter the diagram. The characters are 

 all given correctly to scale excepting that when one exists in less than 1 % of the specimens, it 

 is represented by a line the thickness of which has no relation to the frequency of the same. 

 On the left of the diagram is given the character o, all oculars exsert. This is an invariable 

 feature in the specimens of Echinus affinis observed (p. 118). It is also the character of the 



