396 ROBERT TRACY JACKSON ON ECHINI. 



*Lepidechinus irregularis (Keeping). 

 Plate 63, figs. 1, 2. 



Rhoechinus irregularis Keeping, 1876, p. 37, Plate 3, figs. 6-8; Zittel, 1879, p. 484; Duncan, 1889, p. 



205; Jackson, 1896, p. 200; Tornquist, 1897, p. 754; Klem, 1904, p. 30; Lambert and Thiery, 



1910, p. 122. 



Palaechinus rutoti Julien, 1896, p. 129, Plate 6, fig. 8; Klem, 1904, p. 36; Lambert and Thiery, 1910, p. 119. 

 Palechinus rutoti Tornquist, 1897, p. 753. ' 



Test small, spheroidal, imperfectly known. Ambulacra are narrow, with two columns 

 of low primary plates which imbricate adorally and laterally bevel under adambulacrals. About 

 four ambulacral plates equal the height of an adambulacral. Pore-pairs are uniserial, situated 

 near the outer border of each plate. Interambulacra are broad, with four columns of plates 

 at or about the ambitus, and in the type a trace of a fifth column, as represented by one 

 plate, exists dorsally (Plate 63, fig. 2). These plates imbricate aborally, and from the center 

 laterally and over the ambulacra on the adradial sutures. Externally they bear small secondary 

 tubercles. 



Keeping's holotype is a small specimen which I studied at the Sedgwick Museum, Cam- 

 bridge, England. He naturally made it the type of a new genus (Rhoechinus), as it differed 

 so distinctly from Palaeechinus by the imbrication of its plates. Keeping says of his genus 

 Rhoechinus that it differs from Perischodomus and Lepidechinus in the absence of primary 

 tubercles. This is true in regard to Perischodomus, but was a mistake as regards Lepidechinus, 

 for the type of that genus and other species here attributed to it have only secondary tubercles. 

 It is to be remembered however that Hall's Lepidechinus [Hyallechinus] rarispinus has primary 

 tubercles (p. 293). The species irregularis is clearly referable to Professor Hall's genus Lepid- 

 echinus, so that Rhoechinus becomes a synonym. Professor Duncan overlooked the character 

 of imbrication and used Rhoechinus to include species which he separated from the old genus 

 Palaechinus M'Coy, as earlier discussed (p. 303). 



The specimen described as Palaechinus rutoti by Julien with present evidence cannot 

 be distinguished structurally from L. irregularis. As Julien says, it has two columns of low 

 narrow ambulacral plates, imbricating adorally, with pore-pairs uniserial, and four columns 

 of interambulacral plates which imbricate aborally, and bear small secondary tubercles only. 

 Four ambulacral plates are equal in height to a pentagonal adambulacral plate. The hex- 

 agonal interambulacral plates have a width of 5 mm. and a height of 3 mm. 



Lower Carboniferous, Hook Head, County Wexford, Ireland, holotype in Sedgwick Mu- 

 seum Collection, Cambridge, England, 12; neat Savigny-Poil-Fol, central France (this is the 

 holotype of Palaechinus rutoti Julien, and is in the possession of the family of the late Professor 

 Julien). 





