OCILAH AND GENITAL PLAT! '.:; 



To show the regularity of sequence of incoming of oculur plate- in the- ( Vntrechinoida. 

 it may be put briefly as follows. In the total 48,541 specimens of this order examined and 

 tabulated, 6,235 have all the oculars exsert. In 3, 300 spec! men- there i- one ocular in-ert, and 

 of these in 3,279 cases, it is either ocular I or V, that is, 99.36% are correct by rule. Only 21 

 specimens are aberrant, and these have either ocular IV (text-fin. 140, p. 134) or II in-ert, or in 

 three instances ocular III alone is insert. In 35,184 specimens, two ocular- are in-. -it, and of 

 these it is I and V, the bivium, in 34,881, or 99.14%, are correct by rule. In all of the :{03 

 exceptions, one of the two plates insert is either I or V. In 97 of the-e. ocular- I. IV are inaert, 

 and in 76% of them genitals 4, 5 are fused, as in text-fig. 144, p. 134, mechanically excluding 

 ocular V from the periproct. These may therefore be properly con-iderrd cases of an incom- 

 plete I, V, IV insert. In 56 of the exceptions, oculars I, II are insert (text-fig. 141, p. i:: I 

 This character was found only in the Echinidae or in the nearly allied Strongylocentrotidae ami 

 is a species character in Gymnechinus pulchellus and robillardi (text-figs. 177-179, p. Ui.". In 

 82 of the exceptions, oculars V, IV are insert (text-fig. 142, p. 134). This is especially common 

 in the Arbaciidae, and in no case was an ocular excluded by the fusion of genitals. In ii.'{ cases, 

 oculars V, II are insert. This combination was seen in the Saleniidae, Kchinidae, Strongylocen- 

 trotidae, and Echinometridae, and in all of the cases in the families Kchinidac and Strongylo- 

 centrotidae genitals 5, 1 were fused, excluding ocular I (text-fig. 14<i. p. 134), so that this 

 combination may properly be considered an incomplete I, V, II, which character is common 

 in those families. One specimen has oculars I, III, and four have V, III insert, which are 

 sporadic variants. 



In 2,917 specimens there are three oculars insert. Of these in 2,584, or 88.58%, it is oculars 

 I, V, IV, that is, the bivium and left posterior plate of the trivium. Of the 333 exceptions. 

 in 300 cases the order is I, V, II insert (text-fig. 145, p. 134); that is, the right posterior plate 

 of the trivium is insert instead of the left posterior, as usual. These may be considered as right- 

 handed specimens. This combination was not seen in the order Cidaroida, or in the families 

 Centrechinidae and Arbaciidae where three plates are frequently insert, but it is a feature of 

 the families Saleniidae, Stomopneustidae, Echinidae, and Strongylocentrotidac. It is espe- 

 cially frequent in Sphaerechinus granularis and Tripneustes esculenlus. When thnv plates are 

 insert in the Centrechinoida, it is typically the bivium and usually the left, but occa>i..nally 

 the right, posterior plate of the trivium, and this rule is adhered to in 98.87% of the : 

 cases observed. There were only 33 exceptions to the above. In 17 of these, ocular- V. IV. 

 II are insert, but ocular I was always excluded by the fusion of genitals 5, 1 (text-figs. 1 IS. , , 

 and 196, p. 169), so that this combination may reasonably be considered an incomplete I. V. 

 IV, II insert. In one case oculars I, IV, II are insert (text-fig. 147, p. 134), with genitals 4, 5 

 fused, excluding ocular V, evidently also an incomplete I, V, IV, II insert . In addition, four 

 of the cases are I, V, III insert, a sporadic variant, and eleven are cases of V. IV. Ill iiiM-rt. 



