434 ROBERT TRACY JACKSON ON ECHINI. 



Tholidocidaris irregularis (Meek and Worthen). 

 Plate 73, figs. 3-7; Plate 74, figs. 1-7; Plate 75, figs. 1-5. 



Lcpidocentrus irregularis Meek and Worthen, 1869, p. 78. 



PMidocidaris irregularis Meek and Worthen, 1869, p. 78; 1873, p. 512, Plate 15, figs. 4c^e, 9a-9c; Loven, 



1874, p. 40; Meek, 1874, p. 375; Keyes, 1895, p. 180; Jackson, 1896, p. 210; Tornquist, 1897, p. 760; 



Klem, 1904, p. 22; Lambert and Thie"ry, 1910, p. 123. 

 Pholidocidarw meeki Jackson, 1896, pp. 210, 241, Plate 9, fig. 54; 1899, p. 132; Tornquist, 1897, p. 760. 



This species has been incompletely known, but new material adds considerably to the known 

 facts. This leads me to accept Miss Klem's reference of P. meeki, which I described as a new 

 species, to irregularis as a synonym. No one specimen is complete, but the characters are 

 gathered from different specimens which are later considered separately. 



Test large and spheroidal. In a small specimen, the ventral border of which is about on 

 the plane of the mid-zone (Plate 75, fig. 1), the ambulacra measure 6 mm. in width, the inter- 

 ambulacra 37 mm. in width. Larger specimens greatly exceed this size as gathered from those 

 figured. One imperfect but very large specimen (Plate 74, fig. 2) probably was 100 mm. or 

 more in diameter when alive. 



The ambulacra have apparently only six columns in each area (Plate 73, fig. 3; Plate 74, 

 fig. 1). The ambulacral plates in the ventral and dorsal areas differ very much, a peculiarity 

 known in no other Palaeozoic Echini. Ventrally, the ambulacral plates are very large, the 

 largest measuring about 5 to 8 mm. in width, with the pore-pairs about in the middle of each 

 plate, as seen in area D and partially in area B (Plate 74, fig. 1; also Plate 73, figs. 6, 7). On 

 the other hand, the dorsal ambulacral plates in the same specimen are small, about 3 mm. 

 or less in width, as seen in area B (Plate 74, fig. 1; also see Plate 73, figs. 3, 4). This extreme 

 difference in plates ventrally and dorsally I could hardly have believed if I had not actually 

 found them in place in a single specimen, crushed and distorted enough it is true, yet revealing 

 the facts. There are few plates in the ventral portion of the ambulacrum near the peristomal 

 border (Plate 73, fig. 6), although the exact number cannot be stated, and, passing dorsally, 

 the number of plates and apparently the number of columns increases. 



The interambulacra are no less remarkable than the ambulacra. At the mid-zone and 

 dorsally (Plate 73, figs. 3, 4) there are five or six columns in each area. The plates are scale- 

 like, rounded on the sutures, and imbricate strongly aborally and from the center laterally and 

 over the ambulacra on the adradial sutures. Dorsally, the plates of the adradial columns are 

 very large, being wide, and in height equaling the height of two or three plates of the median 

 columns (Plate 73, figs. 3, 4). Two large isolated adradial plates (Plate 74, figs. 3, 4) measure 

 respectively 23 mm. in height by 15 mm. in width, and 25 mm. in height by 11 mm. in width. 

 These plates show well the ventral and admedian beveled edges which extended under their 



