410 ROBERT TRACY JACKSON ON ECHINI. 



resemblance to the sea-urchins figured by Fraipont (1904) as Oligoporus soreili sp. nov. Frai- 

 pont's specimens, as well as de Koninck's, are from the Carboniferous of Belgium, and on the 

 evidence it seems to me that Oligoporus soreili is a synonym of Proterocidaris giganteus. Act- 

 ing on this opinion, the characters of the genus Proterocidaris are emended by features 

 gathered from Fraipont's publication. 



Test large, spheroidal, or flattened. The ambulacra are narrow, composed at the mid- 

 zone in each area of four columns of low plates which evidently imbricate adorally and laterally 

 bevel under the adradials. Interambulacra are broad, with many columns of plates at the mid- 

 zone. (According to de Koninck's description, there appear to be 65 columns as a whole, and 

 in Fraipont's figures there are from twelve to thirteen columns in each interambulacral area.) 

 The interambulacral plates imbricate strongly aborally and from the center laterally and over 

 the ambulacrals on the adradial sutures. The interambulacral plates bear each a small primary 

 with secondary tubercles and corresponding spines (Plate 65, fig. 3). The apical disc is known 

 only from one genital plate (Fraipont, 1904, Plate 5, fig. 1) , which is low and wide. The peristome, 

 periproct, and lantern are unknown. This genus differs from any of the Palaeechinidae by its 

 imbricating plates and also by its primary tubercles and spines. It differs from other genera 

 of the Lepidesthidae by its four columns of ambulacral plates. It has also many more columns 

 of interambulacral plates than are known in other genera of this family. 



Type and only known species, Proterocidaris giganteus from the Lower Carboniferous of 

 Belgium. 



Proterocidaris giganteus Koninck (emended). 



Plate 65, fig. 3; Plate 67, figs. 4-7. 



Proterocidaris giganteus Koninck, 1881, p. 514, Plate 8; Klein, 1904, p. 74. 



Oligoporus soreili Fraipont, 1904, p. 10, Plate 3, fig. 2; Plate 4, figs. 1, 2; Plate 5, figs. 1, 2. 



Oligoporus giganteus Lambert and Thiery, 1910, p. 121. 



This species is known from de Koninck's publication and also from the description and 

 fine photographic figures given by Fraipont of what he calls Oligoporus soreili sp. nov., but 

 which I consider a synonym as above discussed. Messrs. Lambert and Thiery (1910), con- 

 sider Fraipont's species soreili a synonym of de Koninck's giganteus, but retain the generic 

 name Oligoporus for the species. 



The test is apparently spheroidal, though flattened in the specimens figured. De Koninck 

 says that the diameter of his specimen is exactly 30 centimeters, which represents certainly 

 a huge sea-urchin. Basing additional measurements on Fraipont's Plate 4, fig. 1, which he says 

 is natural size, the test is in this case 160 to 170 mm. in diameter. The ambulacra at the mid- 

 zone, as represented by the periphery of the figure cited, measure about 10 mm. in width and 

 the interambulacra about 85 mm. in width. 



