88 ROBERT TRACY JACKSON ON ECHINI. 



It is interesting to note that in a striking regressive variation in Arbaciu puudidula (Plate 

 4, fig. 11), interambulacrum 4 drops out to a single column of plates in the last four rows built, 

 and dorsally abuts against two oculars, the single colunni and ocular contact being exactly 

 as in Bothriocidaris archaica. A similar condition is seen in Tri]Mieustes (Plate G, fig. 4), 

 Strongylocentrotus (Plate 6, figs. 7, 8), and Eucidaris (text-fig. 185, p. 167). In a unique 

 specimen of Strongylocentrotus drobachiensis (Plate 5, fig. 16) genital 1 lies wholly dorsal to the 

 oculars as do all the genitals in Bothriocidaris archaica (p. 42). 



No pores have been observed in genital plates in Bothriocidaris. It is possible they did 

 not have genital pores, as such are wanting in the young of Recent Echini; more likely they 

 were present, but do not show in external view, and in this they may fairly be compared with 

 Salenia paltersoni (Plate 4, fig. 1) where no pores were seen from the exterior, though they are 

 perfectly visible on the interior of the test (Plate 4, fig. 2). Also no pores have been observed 

 in ocular plates of Bothriocidaris, though in ocular III (Plate 1, fig. 2) there is a peculiar arcuate 

 impression that may function as a pore. 



Neumayr (1881) and Mortensen (1911) consider as genitals in Bothriocidaris the plates 

 lying ventral to the oculars, which I consider as the dorsal plates of the interambulacra. If 

 they are right, then the genitals would be ventral to the oculars, a position opposed to that 

 in all known Echini; also the interambulacra would be separated from contact with the oculars, 

 whereas they are in contact with the oculars in all known Echini. 



Bather (1902), speaking of the periproct of Bothriocidaris, says that the area is filled with 

 minute plates, none of which can safely be selected as the homologues of the genitals of later 

 forms.' If it depended on Bothriocidaris archaica alone, it might be doubtful, but when we 

 see in B. pahleni in part, and in B. globulus in full, the five plates extending down between the 

 oculars so as to come in contact with and cap the interambulacra as genitals do in all later 

 types, it is reasonable to consider these plates as genitals. 



Mr. Agassiz (1892, p. 72) says of Bothriocidaris, "... .we find five radials and five inter- 

 radials forming a single ring round the anal system. At the [dorsal] angles of the radial plates 

 five small anal plates are situated." Again Mr. Agassiz (1904, pp. 79, 80) says, "The genital 

 ring of Bothriocidaris as well as of the Palaechinidae is much like that of the recent echinids, 

 only in the former the ocular plates are far larger than the small plates corresponding to the geni- 

 tals. The anal system of Bothriocidaris globulus Eichw. as figured by Jaeckel ^ shows, as in the 

 very young Cidaridae, five anal plates in the angles of the ocular plates " Mr. Agassiz evi- 

 dently considered as genitals those plates ventral to the oculars as did Neumayr and Mortensen, 

 and those dorsal to the oculars as anals; whereas I consider those ventral to the oculars as the 

 last formed plates of the interambulacra, and the five interradial plates dorsal to the oculars 



'The specimen iiiciilioneil is I lie HoDniociihiriti nrc.haicn sp. nov,, Plate 1, figs. 1, 2, of this memoir. 



