OCULAR AND GENITAL PLATES. 



99 



of species that typically have all oculars exsert. In three specimens (0.4%) ocular V alone 

 is just insert (text-fig. 63) and in one specimen ocular I alone is insert (text-fig. 64), a condi- 

 tion seen often as a first stage of incoming of oculars in development; but as V insert is a 

 frequent variant in the order, probably it typically comes in first in this species. In 3 % oculars 

 V, I are insert (text-fig. 65) . These are all arrested variants. As progressive variants instead 

 of the typical three plates, oculars V, I, IV, II are insert in 8 % (text-fig. 67). This is of interest 

 as these plates are the bivium and posterior pair of the trivium marking the bilateral symmetry 

 of regular Echini. In 4% oculars V, I, IV, III are insert (text-fig. 69). This arrangement of 

 four plates is relatively rare in E. trihuloides while it is a common variant in Cidaris affinis 

 as there discussed. In 25 % all oculars are insert (text-fig. 68). This is the extreme progressive 

 variant of iribiiloides, and is comparable to the typical character of Phyllacanthus baculosa 

 (text-fig. 169, p. 149), or other species in which typically all oculars are insert. In only two 

 specimens out of the 849 examined was the arrangement aberrant. In these variants one has 

 oculars I, IV, III insert, V, II exsert; the other has oculars V, IV, III insert, I, II exsert. Both 

 are sporadic variants and evidently cases of incoinplete V, I, IV, III insert. In studying the 

 specimens they were tabulated in lots of one hundred each, and it is worth noting that the 

 percentage of the typical arrangement antl the variations came out in each lot with hardly 

 any difference, and the same regularity occurred in most species studied. 



In Goniocidaris nutrix (text-fig. 72), typically (86%) the oculars are all fully exsert and 

 a ring surrounds each ocular pore. This ring is a striking character not existent in Goniocidaris 



HI 



Text-figs. 72-74. — Ocular plate arrangement in Cidaridae. 



72. Goniocidaris nuirix (Wyville Thomson). Kerguelcn Island, Antarctic. Diam. 27 nnn. R. T. J. Coll., 743. 

 X 4. All oculars exsert, elevated ring around ocular jjores, genital pores huge. 



73. Goniocidaris canaliculaia A. Agassiz. Falkland Islands. Diam. 24 mm. R. T. J. Coll., 744. X 4. All oculars 

 insert (compare with young, Plate 2, fig. 3). 



74. The same. Falkland Islands. Diam. 22 mm. R. T. J. Coll., 745. X 4. Genitals very low, genital pores 

 reaching beyond plates. 



