92 ROBERT TRACY JACKSON ON ECHINI. 



in the two orders of modern regular Echini, and the sequence of incoming in these orders will 

 now be taken up. 



In the Cidaroida, ocular plates are all exsert in the young, and in adults may be all exsert, 

 or part or all may be insert as a species character (text-figs. 58-74). When oculars become 

 insert, typically V comes in first. It is possible that in some species ocular I may come in 

 before V as both systems occur in the Centrechinoida. After V, ocular I comes in, marking 

 a bilateral symmetry, as these are the plates of the bivium. After the bivium, ocular IV next 

 becomes insert. Up to this point the sequence is as in the Centrechinoida, and V, I, IV insert 

 is a common character in the Cidaroida. Ocular II may come in next (Eucidaris tribuloides) 

 when we have the sequence V, I, IV, II insert, the bivium and posterior pair of the trivium, 

 indicating a bilateral symmetry and being the character that prevails strongly in the Centre- 

 chinoida and Spatangina. Or, on the other hand, ocular III may typically come in after IV 

 (Cidaris affinis), when we have the sequence V, I, IV, III. This is the sequence described 

 by Clark (1907, p. 193) in several genera of the family. It is rare in the Centrechinoida and 

 unknown in the Spatangina. If ocular II comes in first, then III is the last to enter the peri- 

 proct, or if III comes in first, then II is the last to enter the periproct. The sequence as stated 

 in the Cidaroida is adhered to in 99.31% of the 1,459 specimens observed. There were only 

 ten aberrants, or 0.69%. These aberrants all occurred in two species, Cidaris affinis and 

 Eucidaris tribuloides as there described, and all but one of these may be considered cases of 

 incomplete I, V, IV, III (pp. 96, 97, 99.) 



In the Centrechinoida the ocular plates, barring slight variation, come in or reach the 

 periproct in a perfectly definite order. The sequence of incoming is I, V, or V, I, then IV, 

 II, III (text-figs. 118-127). It is to be noticed that this order is the bivium first, then the 

 posterior plates of the trivium, and finally, if at all, the odd anterior ocular. This emphasizes 

 the orientation of Lov6n in comparison with the bilateral Echini, and points out a bilateral 

 symmetry in the regular Echini not before suspected. The bilateral symmetry is shown by 

 the fact that ocular plates come in, or become insert, in definite sequence on each side of the 

 axis through III, 5, which is the elongate axis of bilateral types, as in spatangoids. Thus the 

 posterior plates (the bivium) in the Centrechinoida are the first two to become insert, as in 

 the spatangoid Cassidulus (text-fig. 172, p. 149), next the left anterior ocular comes in, as in 

 Micraster (text-fig. 174), then the right anterior ocular follows, as in Ananchytes (text-fig. 175, 

 p. 149). There is therefore a direct relation between the order of incoming of oculars in regu- 

 lar Echini and that which obtains in the Exocycloida. 



The order of sequence in which the bivium comes in emphasizes family characters. The 

 order I, V is characteristic of the Hemicidaridae, Centrechinidae, Saleniidae, Stomopneustidae, 

 Temnopleuridae, Echinidae, and Strongylocentrotidae (text-figs. 128-134). On the other 

 hand, the order V, I is characteristic of the Arbaciidae and the Echinometridae (the latter 

 as here restricted, see tables pp. 158, 163), text-figs. 111-114, 158-161. 



