LEPIDOCIDARIS 283 



This striking species is one of the most distinctive of Palaeozoic Echini. Test high, spher- 

 oidal, perhaps nearly or quite spherical. In the type (Plate 16, fig. 1) at the upper part of the 

 specimen, which I take to be about on the line of the mid-zone, the ambulacrum measures 

 8 mm. in width; the interambulacrum 52 mm. in width. From these measurements the cir- 

 cumference must have been in the neighborhood of 300 mm. and the diameter about 100 mm., 

 a very large sea-urchin, but no complete specimen is known. In the ambulacrum, every third 

 plate is wider and higher than the intermediate, so that in this character the structure is very 

 different from that of Archaeocidaris. In the type specimen at the mid-zone (Plate 17, figs. 1, 3) 

 the ambulacral plates are rather irregular in form ; while usually all plates cross the half-area, 

 occasional smaller plates may be excluded from the interambulacrum, or the center of the ambu- 

 lacrum, thus making them occluded or demi-plates. Ventrally in the type, and at the mid- 

 zone in younger specimens (Plate 17, figs. 1, 8, 12), all the plates cross the half-area and the 

 large plate of the triplet abuts distinctly against the two smaller plates of the opposite half-area. 

 The ambulacral plates imbricate moderately adorally, and bevel under the adradials. The 

 interambulacrum is very wide, and in the type has eight columns of plates at the mid-zone, but 

 in two smaller specimens (Plate 17, figs. 9, 11) there are only six columns of plates in an area. 

 The interambulacral plates imbricate moderately dorsally and laterally. The character of 

 the tubercles is as described for the genus. The primary spines are cylindrical, with scarcely 

 any perceptible tapering; the largest seen, yet not quite complete, measures 35 mm. in length. 

 The spines are longitudinally finely striate. Small secondary spines occur associated with the 

 secondary tubercles (Plate 17, fig. 10). The pyramids are wide-angled, foramen magnum 

 moderately deep, lateral wings with ridges for the attachment of interpyramidal muscles, 

 all practically the same as in Archaeocidaris. 



This species is known only from the Lower Burlington Limestone, Lower Carboniferous, 

 Burlington, Iowa. The holotype is in the Museum of Comparative Zoology Collection 3, 026; 

 also a number of other specimens are in the same collection. A fine series of twelve specimens 

 is in Mr. Frank Springer's collection. 



In the type and in part in other specimens, the introduction of the interambulacral columns 

 is shown. As seen in Plate 17, fig. 9, there are four columns of plates in an interambulacral area 

 ventrally, and apparently four in the basicoronal row, as in Archaeocidaris. Passing dorsally, 

 new columns are introduced with a pentagonal plate, and a heptagonal plate lies ventral to the 

 same. On the basis of introduction of columns and also on the direction of imbrication, I have 

 oriented the type as shown in Plate 16, fig. 1, and Plate 17, fig. 1. This is reversed from the 

 position given by Meek and Worthen. 



