102 ROBERT TRACY JACKSON ON ECHINI. 



exsert in the very j'oung (Plate 2, fig. 3), as in tlie Jurassic. In a series of eight j'oung speci- 

 mens from Patagonia in the American Museum of Natural History, varying from 15 to 20 mm. 

 in diameter, the oculars are slightly or more strongly exsert in all but one, the largest, in which 

 ocular V alone is insert. The genitals are high in the young rather than low as in the adult, 

 therefore more like typical cidarids, and all or jjart of the genital pores are wanting in the smaller 

 specimens. This all indicates that adult characters are taken on late in this species, indeed 

 much later than usual in Echini. In adults the genitals are relatively the lowest, especially 

 those shown in text-fig. 74, of any genitals seen in Echini; perhaps in some way correlated with 

 the peculiar brooding habit of this species. Mr. Agassiz (1873, p. 396) said of G. canaliculata 

 that the genital plates are in contact. He included (1881, p. 44), however, Gonioddaris nutrix 

 in this species, and it is possible that his observation of exsert oculars was based on nutrix, 

 in which it is characteristic. As I find six out of seven adults with all oculars insert, it is fair 

 to assume that as the specific character. In Gonioddaris biserialis all the oculars are insert 

 in three specimens. 



In Phyllacanthus there is wide range in species as to the insertness of oculars, as shown 

 in the table; but with one exception only a few specimens were studied. In. Phyllacanthus 

 imperialis oculars are typically all exsert. In P. thomasii, in 33%, oculars V, I, IV, or all, 

 are insert, but one specimen has all exsert and one has V only insert, these last two being the 

 largest specimens of the species seen. In P. annulifera oculars are typically all insert, as seen 

 in 53 % of the 17 specimens. Of arrested variants 6 % have ocular V only insert, 18 % have I, 

 V, IV, 12% I, V, IV, II, and 12%, I, V, IV, III insert. In these characters the species is dis- 

 tinctly more primitive than the next. In Phyllacanthus baculosa (106 specimens) the oculars are 

 all insert in 98 % of the cases (text-fig. 169, p. 149). In one exception oculars V, I, IV are insert 

 (Plate 3, fig. 3), for this species an arrested variant, comparable to the typical character of Euci- 

 daris tribuloides (text-fig. 66). In another specimen oculars V, I, IV, II are insert and III 

 exsert (Plate 3, fig. 4). This arrested variant is especially interesting as it is like the progressive 

 variant of Euddaris tribuloides (text-fig. 67), and also as it indicates that III is the last ocular 

 to become insert as in that species and in the Centrechinoida. Phyllacanthus baculosa with all 

 oculars insert is in this feature like the majority of Palaeozoic species, and like progressive 

 variants of those in its own order that typically have fewer oculars insert. It is the only species 

 of Echini with all insert typically in which a considerable number of specimens have been 

 studied, and it is interesting to see that its variants, though so few, accord with the characters 

 of associated types. 



Considering the ocular arrangement in the Cidaroida as a whole, it is seen that they follow 

 very closely the order of insertness that is characteristic of the group, but yet show a very 

 wide diversity of arrangement within the species. The variants skip about within the limits 

 of variation in a degree that is unknown in the Centrechinoida, yet aberrant variants are 



