414 ROBERT TRACY JACKSON ON ECHINI. 



are both ambulacral and non-ambulacral plates on the peristome, while in the Echinothurii- 

 dae there are ambulacral plates only (text-figs. 43, 47, p. 80). I do not know the character 

 of the peristome in Pholidocidaris (p. 433), but presume that like Lepidesthes (Plate 68, fig. 3) 

 it had ambulacral plates only, and in so far both genera may be compared with the Echino- 

 thuriidae as regards the structure of the peristome. I do not see that a similarity of the 

 peristome, where existent, necessarily implies a similarity of the apical disc, as assumed by Mr. 

 Agassiz. The character of the apical disc in the Echinothuriidae is to have oculars all insert, 

 genitals split by secondary sutures, and very commonly for oculars and genitals to be separated, 

 so that the periproct comes in contact with the interambulacra (text-fig. 170, p. 149; also pp. 

 63, 110, 168, 213). This last feature, of separation, apparently was the most important one 

 in Mr. Agassiz's mind. Mr. Agassiz (1904, p. 30, Plate 11, figs. 1,5,6) shows that in Poroci- 

 daris cobosi the peristome may be covered with ambulacral plates only, yet the apical disc 

 with exsert oculars is very different from that of the Echinothuriidae (p. 110). In the apical 

 disc of the Perischoechinoida oculars are usually all insert (p. 89) ; I know of no case of split 

 genitals, and oculars and genitals form a closed ring in all cases I know of with the possible 

 exception oi Koninckocidaris silurica (pp. 286, 287). I do not know the structure of the apical 

 disc in Archaeocidaris (pp. 265, 266), though I infer from a statement on another page of the 

 work cited, that Mr. Agassiz (1881, p. 80) did. The apical discs in Lepidesthes (text-fig. 251, 

 p. 428; Plate 68, fig. 5), and Pholidocidaris (Plate 73, figs. 3, 4) as far as known do not bear 

 any close resemblance to that structure in the Echinothuriidae. 



In discussing the anal system, Mr. Agassiz (1881, p. 11) says, "The figures of Bailey, and 

 of Meek and Worthen, and a specimen of Lepidesthes w^hich I have had occasion to examine, 

 would seem to indicate a splitting up of the central plate into a great many smaller plates." 

 Baily (1865b, Plate 4, fig. B) figured the periproctal plates of Palaeechinus elegans (p. 309), 

 but I believe neither he nor any one else has shown a central (suranal) plate in Palaeozoic species 

 (pp. 174, 176). As regards the origin of periproctal plates by splitting in Lepidesthes as 

 assumed by Mr. Agassiz, I can only say that I have seen no evidence for the formation of peri- 

 proctal plates by splitting in this or an}- other genus of Echini. It appears rather that peri- 

 proctal plates originate as separate centers of calcification (text-figs. 59-170). Mr. Agassiz 

 (1874, p. 642) says that the compound ambulacral plates of the Centrechinoida are formed by 

 splitting, whereas they are clearly formed by the coalescence of originally simple plates as 

 discussed (pp. 55, 56). He also considered that interambulacral plates in the Echinothuriidae 

 are derived from the periproct and he assumed the same view for certain of the Perischo- 

 echinoida (A. Agassiz, 1881, p. 80; 1883, p. 33). As Mr. Agassiz (1883, p. 32) expresses it, 

 "The new interambulacral plates are found to be pushing out from the plates of the anal system 

 on each side of the genital plates." Or as he says later (A. Agassiz, 1904, p. 95), "That this 

 remarkable intercalation exists there is not the slightest doubt, and it naturally suggests in old 



