430 ROBERT TRACY JACKSON ON ECHINI. 



In interambulacrum A there are four columns of plates from the ventral portion as far as pre- 

 served to the mid-zone; but above the ambitus, and 14 mm. below the top of the specimen, a 

 fifth column appears in the middle of the area and continues to the dorsal part of the test. 

 This is the only case of more than four columns seen in the species. The interambulacral 

 plates are worn, but bear small secondary tubercles and spines (Plate 70, fig. 4) . 



Another very large specimen in the Museum of Comparative Zoology is remarkably clearly 

 preserved (Plate 69, fig. 8). The ambulacra are relatively very wide, as shown in the figure 

 and by the measurements given above. The ambulacral plates at the mid-zone are relatively 

 wide hexagons, as in the last specimen described, and, again like it, the plates dorsally are 

 rhombic in outline instead of hexagonal. The interambulacra have four columns of plates in 

 areas A and C, and in both the plates of column 3 are wide, imbricating laterally in two direc- 

 tions. Ambulacral and interambulacral plates show well the numerous small secondary 

 tubercles. 



*Lepidesthes extremis sp. nov. 



Text-fig. 252, p. 431; Plate 71, fig. 2; Plate 72, figs. 1, 2; Plate 73, figs. 1, 2. 



This interesting species is represented by two specimens which were kindly loaned me for 

 study by Mr. Eber Hyde, of Lancaster, Ohio, through his son, Mr. J. E. Hyde, of Columbia 

 University. The specimens are internal or external molds in a dark siliceous horn-stone, 

 stained a yellowish brown by iron. This species, while known only from portions of the test, 

 and those not at the mid-zone, yet represents the most massive test and proportionately the 

 widest ambulacra known in this striking genus. The fullest idea of the species is gained from 

 the specimen which is an internal mold. This specimen, as far as preserved, measures 93 mm. 

 in width by 45 mm. in a plane at right angles to the same. As it is somewhat compressed and 

 not complete on the periphery, it is difficult to estimate the size, but in the ventral view, Plate 

 72, fig. 1, from the center to the outer limit of ambulacrum H, it measures about 55 mm. 

 From this radial measurement the test when alive probably measured not less than 100 mm. in 

 diameter. One side of the specimen bears an impression of the ventral portion of the test, and on 

 the reverse side a similar impression of parts of the dorsal portion of the test (Plate 72, figs. 1, 

 2; Plate 71, fig. 2). No portion of the specimen extends far enough to give the character at the 

 mid-zone, and the ventral impression is the most instructive. In this view and at a zone which 

 is much below the mid-zone (Plate 71, fig. 2) ambulacrum B measures 33 mm. in width in a 

 straight line across from its points of aboral contact with interambulacra A and C. Interam- 

 bulacrum C measures at its aboral border (Plate 71, fig. 2) 5 mm. in width; therefore the am- 

 bulacrum at this zone is something more than six times the width of the interambulacrum, a 

 proportion in excess of that known in any other regular sea-urchin, living or fossil, and in 

 this character approached only by Meekechinus elegans (p. 443). The ambulacrum in Echini is 



