210 ROBERT TRACY JACKSON ON ECHINI. 



(Plate 1, fig. 1), in the corona each ambulacrum consists of two cokmins of high hexagonal 

 plates, each with two pores, superposed, surrounded by a peripodium, and situated in the middle 

 of the plate. Such a structure is not known in any other adult regular echinoid, but is almost 

 identical with that seen in young Goniocidaris (Plate 2, figs. 1-3), Strongylocentrotus (Loven, 

 1892), and Phormosoma (Mortensen, 1904, p. 54). Each interambulacrum of Bothriocidaris 

 consists of a single column of plates, which is represented by a single plate at the ventral border 

 of the interambulacra in the young of all other Echini (Goniocidaris, Plate 2, fig. 1 ; Strongylo- 

 centrotus, Plate 3, fig. 11; Phormosoma, Plate 3, fig. 10). This single plate is also seen in 

 adults where resorption has not removed the base of the corona, as in the Palaeozoic Hyattechinus 

 rarispinus (Plate 23, fig. 1), H. pentagonus (Plate 25, fig. I), H. beecheri (Plate 26), Perischo- 

 domus (Plate 64, fig. 2). Again, the single ventral plate is seen in adult Recent Phormosoma 

 (text-fig. 43, p. 80), Arbacia (text-fig. 227, p. 193), in the Triassic Tiarechinus (text-fig. 31, 

 p. 70), and in nearly all clypeastroids and spatangoids (text-figs. 27-29, p. 70, and 52, 54, p. 80; 

 Plate 3, fig. 15). It seems that there is ample proof that the interambulacrum begins with a 

 single plate, as shown by Loven (1874), and Mortensen (1903) in Hypsiechinus (Plate 3, fig. 6). 

 This structure with less evidence I correlated (Jackson, 1896, p. 233) as a stage in development 

 with the single column of plates in Bothriocidaris, naming it the protechinus stage. As 

 Palaeozoic types with many columns of interambulacral plates begin at the ventral border, the 

 young, with a single plate representing a single column, and later add their several columns 

 during development (Plate 26), it seems that Bothriocidaris throws great light on the numerous 

 columns there existent (p. 64). 



In Bothriocidaris a row of primordial ambulacral plates forms a continuous ring around 

 the mouth, as in all succeeding regular Echini, and they are relatively large, as in the young of 

 modern Echini, Goniocidaris (Plate 2, fig. 1). In Bothriocidaris a second row of plates sur- 

 rounds the first, completing the peristome. This second row was probably derived by flowing 

 down from the corona, and is closely comparable to the second stage in development of Phor- 

 mosoma (Plate 3, fig. 10), as pointed out by Mr. Agassiz (1904, p. 79). A striking feature 

 of Bothriocidaris is the great size of the oculars and the very small genitals (Plate 1, fig. 2). 

 This may fairly be compared with the similar condition in very young Echinus microtubercu- 

 latus (Plate 3, fig. 5), as shown by Bury (1895), though the adult of the same species has large 

 genitals which exclude the oculars from the periproct. The most difficult thing to account 

 for in Bothriocidaris is the fact that the teeth seem to be radial in position, as discussed (p. 242). 



Of the three species of Bothriocidaris, the salient features are, briefly, as follows. In B. 

 archaica sp. nov. the oculars meet in a continuous ring with genitals dorsal to the same (Plate 

 1, figs. 1, 2), interambulacral plates bear spines. In B. pahleni Schmidt (Plate 1, figs. 3-6), 

 part of the genitals lie dorsal to the oculars, and part separate the oculars, reaching the inter- 

 ambulacra; interambulacral plates are without spines. In B. globulus Eichwald (Plate 1, figs. 



