438 ROBERT TRACY JACKSON ON ECHINI. 



plates in the two areas preserved, but in area A column 5 and in area C column 6 drop out early, 

 above which five columns extend to the apical disc. A small plate with two pores above am- 

 bulacrum B may be an ocular; if so, it is the only ocular yet recognized in the genus (p. 433). 

 A genital above interambulacrum A is a large, rounded plate with ten genital pores. 



A small series of isolated interambulacral plates from near Burlington, Iowa, in the Univer- 

 sity of Michigan Collection, is instructive as they are sharply and clearly preserved (Plate 74, 

 figs. 3-5) . The plates are all from adradial columns as indicated by the presence of a primary 

 tubercle on each, and the beveled edges extending adorally and admedially demonstrate that 

 figures 3 and 5 are from a left adradial and figure 4 from a right adradial column. The plates 

 shown in Plate 74, figs. 3, 4, are high and wide, and would belong to the dorsal portion of a test. 

 The plate shown in Plate 74, fig. 5, is low and wide; it could not be a young plate dorsally, as the 

 primary tubercle is well developed, and it may have come from about the mid-zone or farther 

 adorally. A right half-pyramid (Plate 74, fig. 6) indicates that the pyramid was wide-angled 

 with moderately deep foramen magnum ; the suture for the epiphyses is seen dorsally. Another 

 half-pyramid seen from within (Plate 74, fig. 7) shows that the lateral wing is wide, indicating 

 an inclined lantern (compare Plate 12, figs. 6-8). This specimen shows the pyramidal suture 

 and dental slide for the support of the tooth, the slide failing to reach the base of the foramen 

 magnum as usual in the Perischoechinoida (compare Plate 2, fig. 10). 



An important, but somewhat imperfectly preserved specimen, showing the ventral part 

 of the test, is in the Museum of Comparative Zoology. This specimen is from Warsaw, Illi- 

 nois (Plate 73, figs. 6,7), and I described it (1896, p. 210) as the type of a new species, Pholi- 

 docidaris meeki. I did not then recognize the differences that exist between the ventral and 

 dorsal parts of the test, so that it was felt the differences warranted specific distinction. Miss 

 Klem (1904, p. 23) treated meeki as a synonym of irregularis, and I quite agree with her. In 

 my earlier publication errors were made which require correction. On page 211 (of Jackson, 

 1896) it is stated that the plates of both ambulacral and interambulacral areas imbricate ador- 

 ally. This is true of the ambulacral plates, but interambulacral plates imbricate aborally, 

 and from the center outward and over ambulacrals on the adradial suture as usual (discussion, 

 p. 76). Another error was made in the drawing of Plate 9, fig. 54, in which I showed column 4 

 as originating in the fourth row, whereas it does not originate until two rows later, as seen in 

 the present figure. In this specimen (Plate 73, figs. 6, 7) the ambulacral plates are very large, 

 somewhat irregular in shape, but approaching rhombic in form. Dorsally, on the aboral 

 border, some small ambulacral plates appear, having taken on the character typical of all the 

 plates dorsally. The pore-pairs are about in the middle of each plate in a raised area, and 

 resemble the holes in a button. The ambulacral plates imbricate adorally and laterally bevel 

 under the adradials. The specimen is too much worn to show secondary tubercles, which alone 

 probably existed on the ambulacral plates. As briefly considered above, the primordial inter- 



