PLATE 61. 

 Melonechinus vanderbilti sp. now Page 38S. 



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 interambulaerum drawn from 

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

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



Fig. 2. Interambulaerum A. X 0.0. Drawn largely from an internal mold of the plates, but a few platen arc in place 

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

 are 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 lias a compensating heptagonal plate, H, on its right side. Column 

 6 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 

 S at the points P', P", so that above this zone there arc onlj five columns of plates in the area, indicating a senescent 

 individual. 



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

 plate; tubercles are 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- 

 erum in each plate, tubercles on interambulacral plates. 



Fig. 6. 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 interambulaerum 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, 391) i. 



Fig. 8. Segment across a half-ambulacrum and completely across an interambulaerum, 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 (ilemi-) 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.) 



