PHOLIDOCIDARIS. 437 



of irregularis, it seems safest to ascribe the differences to immaturity. In thifi very choice 

 specimen (Plate 73, fig. 4) ambulacral plates are in place in all five areas. Doreally in area B, 

 also in area D, there are only four columns. The ambulacral plates are all very small, like the 

 dorsal plates in Plate 74, figs. 1, 2. The interambulacral areas are in part well preserved, and 

 are by all means the most nearly complete of any known specimen. The adradial plates in 

 each area are wide, very high, and about the height of two to three plates of the median column-. 

 The adradials bear an eccentric primary with secondary tubercles, the median plates secondary 

 tubercles only, though for the most part these have been omitted in the drawing. They can 

 be seen, however, in the photographic figure (Plate 75, fig. 2), and are shown in the drawing 

 of another specimen (Plate 73, fig. 3). In interambulacrum A there are six columns of plates, 

 the sixth coming in above the adoral border and represented by only three plates, above which 

 to the apical disc there are only five columns (Plate 73, fig. 4). In area I a sixth column is also 

 represented by some plates. In area C there are only five columns, all of which extend through- 

 out the area as far as preserved. Area E is doubtful, but there are apparently five columns in 

 area G; of course, ventrally in this area, if preserved, there might have been a sixth column 

 represented as in area A. A genital plate is in place, capping area A. It is a low, broad, rounded 

 plate, with seven pores. Another genital, capping I, but somewhat displaced, shows only five 

 pores. No ocular or periproctal plates were recognized. 



Another immature specimen, quite similar to that just described, is from Keokuk, Iowa, 

 in Mr. Frank Springer's collection. This specimen (Plate 75, fig. 1 ; Plate 73, fig. 3) has parts 

 of three ambulacral and two very good interambulacral areas. The adoral border is about on 

 the line of the mid-zone. The ambulacral plates are all small, and in area D there are three 

 columns of plates which evidently belong to the right half-area, and thus represent six columns 

 for the whole area at this zone. In both interambulacral areas the adambulacral plates are 

 wide and very high, equaling the height of two to three median plates in the same zone. 

 It is to be noted that this great size and especially the great elongation of adradial plates are most 

 marked in the three or four more dorsal plates of each column, and near the mid-zone the adra- 

 dial plates are much lower, as seen in Plate 73, figs. 3, 4, and Plate 75, fig. 3. On the ventral 

 side of another specimen (Plate 73, fig. 6) it is seen that in area C the plates of the adradial 

 columns are no wider nor higher than are the plates of median columns. It is evident therefore 

 that this specialization of adradial plates is a character taken on late in development and occurs 

 only in the dorsal portion of the test. The adradial plates have a large eccentric perforate 

 primary tubercle and numerous small secondary tubercles (Plate 73, fig. 3), except that the one 

 youngest plate next the apical disc in each adradial column has not yet acquired the primary 

 tubercle. This is a general character of youthful plates in the placogenous zone (compare 

 text-figs. 92, 94, and 96, pp. 106, 107, 108; Plate 11, fig. 2). The plates of the median columns 

 have secondary tubercles only (Plate 73, fig. 3). There are six columns of interambulacral 



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