PLATE 61. 



Melonechinus vanderbilti sp. nov. Page 388. 



Figs. 1-4. Same specimen as photograph, Plate 58, fig. 5. Holotype. 



Fig. 1. Section dorsal to the mid-zone. X 1.9. A half-ambulacrum in surface view, and an interambulacrum drawn from 

 the internal mold of the base of the plates. Six columns of plates in a half-area of an ambulacrum, uiul nine 

 columns of plates in an interambulacral area. Ambulacral plates have tubercles and peripodia. 



Fig. 2. Interambulacrum A. X 0.9. Drawn largely from an internal mold of the plates, but a few plates arc in place 

 as indicated by those having tubercles, partly restored ventrally and dorsally as indicated by dotted lines. There 

 :ire three plates in the second row, four plates in the third row, column 5 originating in the seventh row. An excep- 

 tional pentagon, P, in the fifth row of column 5 has a compensating heptagonal plate, H, on its right side. Column 

 (i originates on the left of the center in the tenth row, column 7 in the 14th row, column 8 in the 20th row, and col- 

 umn 9 in the 29th row. Curiously, in both columns 8 and 9, there is an exceptional pentagon, P, with adjacent 

 heptagons similar to that seen in column 5. Dorsally columns 1 and 2 drop out at the points X and columns 5 and 

 8 at the points P', P", so that above this zone there are only five columns of plates in the area, indicating a senescent 

 individual. 



Fig. 3. Surface detail. X 2.8. Pore-pairs arc in peripodia and lie toward the next adjacent interambulacrum in each 

 plate; tubercles :trc represented on interambulacral plates. 



Fig. 4. Inner detail of an ambulacrum drawn from an internal mold. X 2.8. Pore-pairs are in the middle of the plates 

 of the inner columns of isolated plates, or toward the median line of the half-area in the lateral ambulacral column 

 of demi-plates. Ambulacral plates are fan-shaped opposite the horizontal interambulacral sutures. 



Melonechinus giganteus (Jackson). Page 389. 



Figs. 5-9. St. Louis Group, Lower Carboniferous, Clarksville, Tennessee. U. S. Nat. Mus. Coll., 42,340. 



Fig. 5. Detail of ambulacrum from the exterior. X 2.8. Pore-pairs are in peripodia toward the adjacent interambula- 

 crum in each plate, tubercles on interambulacral plates. 



Fig. 1). Same individual plates seen from the interior. X 2.7. Pore-pairs of the median isolated columns are in the mid- 

 dle of each plate, but pore-pairs of the column of demi-plates bordering on the interambulacrum are in the inner 

 border of each plate. Adambulacral plates are rounded on the adradial suture. 



Fig. 7. Profile view of plates shown in figs. 5 and 6. X 2.7. The plates marked X and XX locate the individual plates of 

 the several figures (pp. 360, 390). 



Fig. 8. Segment across a half-ambulacrum and completely across an interambulacrum, to show surface character. X 2.7. 

 There are six columns of plates in the right half-ambulacrum and eleven columns of interambulacral plates, the 

 eleventh appearing in this view, which is about on the plane of the mid-zone. Peripodia are shown in ambulacral 

 plates and tubercles on the same, and on a few interambulacral plates. (Compare text-fig. 237, p. 231.) 



Fig. 9. Segment showing structure viewed from the interior. X 2.7. Pore-pairs are in the middle of all isolated am- 

 bulacral plates; but in the plates of the median (occluded) and lateral (demi-) columns the pore-pairs in each plate 

 are toward the middle of the half-area. Ambulacral plates are fan-shaped opposite the horizontal interambulacral 

 sutures (p. 60). (Compare Plate 56, fig. 5.) 



