398 ROBERT TRACY JACKSON ON ECHINI. 



in the holotype (Plate 63, fig. 6) one column, either 5 or 6, drops out dorsally with a pentagonal 

 plate, indicating senescence. The interambulacral plates are curved on the outer surface and 

 bear secondary tubercles only. They are slightly rounded on the suture lines and imbricate 

 moderately aborally and from the center laterally and over the ambulacra. Adambulacral 

 plates are pentagonal, rounded on the adradial sutures (compare Plate 64, fig. 3). The plates 

 of the median columns are hexagonal. On the interior of the test (Plate 62, fig. 3; Plate 63, 

 fig. 7) the interambulacral plates are more sharply angular than on the exterior, and adambula- 

 cral plates are not rounded on the adradial sutures (compare text-figs. 32, 34, p. 75). 



The ventral portion of the test is unknown. Dorsally, as seen from within (Plate 62, fig. 

 3; Plate 63, figs. 7, 8) the apical disc is in place; it is small, measuring 9 mm. in diameter. The 

 whole of this specimen is not known but it corresponds nearly enough with the holotype so that 

 it was at least fully as large. On this basis the diameter of the apical disc as seen from within 

 would be proportionately about 18 % (or less) of the diameter of the test. Several ocular and 

 genital plates are in place. The oculars are triangular, fully exsert, each being shut out from 

 the periproct by the contact of the two adjacent genitals, a rare character in the Palaeozoic, but 

 like the dominant Mesozoic and the youthful character of modern Echini (p. 89). Each ocular 

 has one pore near the ventral border, but as this is an internal view, it is not certain that the 

 pore reached the surface (p. 89). The oculars ventrally cover the ambulacra and laterally 

 in part the interambulacra on either side, the young interambulacral plates being in contact 

 with the oculars (pp. 62,86). This is the specimen to which Mr. Agassiz (1904, p. 80) referred 

 as Lepidechinus imbricatus, and in which he noted that th.e young interambulacral plates origi- 

 nate against the oculars. The genital plates are high and wide, and, as far as preserved, meet 

 in a continuous ring dorsally on the periproctal border. Each genital has a single pore near the 

 middle of the plate. This single pore in each plate is the only case with this character seen in 

 the Palaeozoic, and also the median position is unusual; but it should be remembered that 

 this is an internal view, and in an external view the pores may be nearer the ventral border 

 (p. 171). Genital pores are usually near the ventral border of the plates, but they may be near 

 the middle of the plates in external view, as seen in Dermatodiadema (text-fig. 80, p. 104) and 

 Peltastes wrighti (Plate 4, fig. 7). The plates of the periproct and the lantern are unknown 

 in this species. 



This species differs from Lepidechinus iowensis in having six instead of five columns of 

 plates in an interambulacral area. It differs from Lepidechinus imbricatus in having six instead 

 of eight columns of plates in an interambulacral area, and also in that the plates are rela- 

 tively larger, heavier, and less strongly imbricating. 



Upper Burlington Limestone, Lower Carboniferous, Burlington, Iowa, holotype, Museum 

 of Comparative Zoology Collection 3,053; paratype, same museum, no. 3054; second paratype, 

 F. Springer Collection 8,011. 



