252 ROBERT TRACY JACKSON ON ECHINI. 



plates on the peristome are a typical echinoid character. The inclined lantern is of the typical 

 character seen in Palaeozoic species, is purely echinoid, and has no relation to the structure 

 of any known starfish. There are grooved teeth, narrow epiphyses, braces, and even rather 

 deeply forked compasses as shown by Sollas (1899, text-figs. 7, 8) whose figure 7 is in part 

 reproduced in my Plate 18, fig. 5. Some knowledge of ocular and genital plates is much to be 

 desired, and the position of the anus should be more positively settled. If Spencer is correct 

 in showing the anus in an interambulacral area, we must consider Palaeodiscus a peculiar and 

 irregular echinoid. If the anus is in the center dorsally, as Gregory says (1897, p. 129), then 

 Palaeodiscus is not irregular, is wrongly placed in this order, and could properly find a place 

 in the Lepidocentridae, from which it differs essentially in this character only. 



Family ECHINOCYSTIDAE Gregory. 



Echinocystidae Gregory, 1897, p. 133; Lambert and Thiery, 1910, p. 118. 



Test spheroidal, ambulacra narrow with at the mid-zone four columns of plates which are 

 demi- and occluded. Interambulacra broad with many columns of irregular plates. Plates 

 thin with small primary and secondary tubercles and spines. Apical plates doubtful, but the 

 anus is eccentric in an interambulacrum. A well developed typical echinoid lantern exists. 

 One genus and species. 



ECHINOCYSTITES Wyville Thomson. 



Echinocystites Wyville Thomson, 1861, p. 106; Duncan, 1889a, p. '20; non Hall, 1865; 1868a, p. 316. 



Cystocidaris Zittel, 1879, p. 480. 



Echinocystis Gregory, 1897, p. 124; Lambert and Thiery, 1910, p. 118. 



With characters of the family. The type-species is E. pomum Wyville Thomson, from 

 the Silurian of England. 



"Echinocystites pomum Wyville Thomson. 

 Plate 16, fig. 4; Plate 18, figs. 6-8; Plate 20, figs. 1-1. 



Echinocystites pomum Wyville Thomson, 1861, p. 109, Plate 3, figs. 1-3; Plate 4, figs. 1-5. 



Echirwcystites uva Wyville Thomson, 1861, p. 110, Plate 4, figs. 4,5. 



Cystocidaris pomum Zittel, 1879, p. 480. 



Cystocidaris uva Zittel, 1879, p. 480. 



Echinocystis pomum Gregory, 1897, p. 124, text-figs. 1-3, Plate 7, fig. 4; Sollas, 1899, p. 707, text-figs. 



12-14; Klem, 1904, p. 13; Lambert and Thiery, 1910, p. 118. 

 Echinocystis [pomum] Spencer, 1904, pp. 31, 34. 

 Echinocystis uva Lambert and Thiery, 1910, p. 118. 



The test is spheroidal, ambulacra narrow with four columns of plates in an area, demi- 

 and occluded, pore-pairs are biserial and pass through about the center of each plate. Inter- 



