LEPIDESTHES. 425 



"Lepidesthes colletti White. 



Text-figs. 14, p. 54; 21, p. 59; 32-38, p. 75; 251, p. 428; Plate 69, figs. 1-8; I'lati- 70, figs. 1-5; Plate 71, fig. 1. 



Lepidesthes colletti White, 1878, p. 33; 1880a, p. 163, Plate 40, figs. 2a, 2b; 1882, p. :iii.', I'lat,- 1 1 . fis. 2, :i; 

 Keyes, 1895, p. 184; Jackson, 1896, pp. 210, 241; Mem, 1904, p. 24; Laml-rt ,-m.l Tl.irry, l!in. 

 p. 123. 



This species is one of the most striking known from the Palaeozoic, and I have had the 

 opportunity of studying twelve fine specimens, including one figured by Dr. White (1880a, 

 Plate 40, figs. 2a, 2b) which was kindly loaned me by Professor Stuart Weller; also a fine 

 series formerly in my collection, and now in the Museum of Comparative Zoology, or the Brit i>h 

 Museum, and specimens in Mr. Braun's and Mr. Springer's collections. 



The test is high and elliptical. When Dr. White originally described thi< >peeic> in 1S7S, 

 he had only a single specimen which he says was in a crushed condition, the height about \~> 

 mm. and the diameter considerably less. In Dr. White's second paper in which tlii> >pccie> 

 is described, he figured a second specimen which was not the type, and is considerably smaller, 

 as he noted. This second specimen (Plate 69, figs. 2, 3) measures about 37 mm. in height 

 and 35 mm. in width. The ambulacra at the mid-zone measure 12 mm. in width and the inter- 

 ambulacra 5 mm. in width. Another specimen in the Chicago University Museum, which i- 

 larger and more nearly complete (Plate 69, fig. 4), measures 47 mm. in height and 40 mm. in 

 width. The ambulacra at the mid-zone measure about 15 mm. in width and the interambulacra 

 7 mm. in width. A slightly larger specimen in the Museum of Comparative Zoology (Plate 

 69, fig. 6) is somewhat flattened, but otherwise without distortion. It measures about 51 mm. 

 in height and 40 mm. in width at the mid-zone. The ambulacra at the mid-zone measure about 

 14 mm. in width and the interambulacra 7 mm. in width. A very large specimen in the Museum 

 of Comparative Zoology (Plate 69, fig. 8) measures about 70 mm. in height and 65 mm. in width. 

 The ambulacra measure about 26 mm. in width and the interambulacra 9 mm. in width. The-e 

 measurements show that the ambulacra are twice as wide as the interambulacra or wider, and 

 are proportionately wider in large specimens than in smaller ones. They are, however, cer- 

 tainly never five or six times as wide, as stated by Dr. White (1880a, p. 163). 



The ambulacra are very wide, at the mid-zone composed of 16 columns of plates. In no 

 case have I found 18 or 20 columns in an area, as Dr. White thought possible. The structure 

 consists of a column of occluded plates extending from the middle of the area outward, a column 

 of demi-plates extending from the interambulacra inward, and six regular columns of isolated 

 plates in each half-area. This is as highly evolved an ambulacral structure as is known in any 

 Palaeozoic sea-urchin excepting Meekechinus ekgans, in which there are 20 columns in an area 

 at the mid-zone (Plate 76, fig. 1). The ambulacral plates imbricate strongly adorally and 



