Explanation of Plate 5. 



Figure 20. — Melonites multiporus ; showing columns of plates in 2 interambulacral 

 areas and the rhombic form of newly introduced plates dorsally. 

 The arrangement of the columns and plates is nearly normal through- 

 out. The ambulacral area should be compared with plate 2, ligure 

 4, for the relations between the structure of the ventral border and 

 the am])itus ; also with Oligoporus, plate 6, figures 25 and 30 ; also 

 compare with figure 1, page 191, for the relations expressed in that 

 more primitive genus. Saint Louis group, Subcarboniferous, Saint 

 Louis, Missouri. Specimen in Museum of Comparative Zoology, 

 catalogue number 3005. Life size. Pages 149, 153. 



Figure 21. — Melonites giganteas, Jackson. Interambulacrum showing arrangement 

 and method of introduction of 11 columns of plates. Column 4 

 originates to the left of the center (compare table,- page 180), but 

 the otlier 8 columns added all originate in the theoretically correct 

 position. Below pentagon 9 is a tetragonal plate, which is really the 

 first plate of the ninth colunm. Associated with this jjlate are a 

 hexagon, A, and 2 accessory heptagonal plates, IF and H^^ (com- 

 pare with ninth column in the four other areas, table, page 180). 

 The eleventh column drops out before reaching the dorsal termi- 

 nation of the area. Associated with the two last formed tetragonal 

 plates of the eleventh column are 4 heptagonal plates, //, and 2 

 octagonal plates, 0, which compensate for loss of sides in the tetrag- 

 onal plates. Kecently added plates in the dorsal area are more or 

 less rliombic. Com[)are with tabulation of the specimen, page 180, 

 of which this figure represents area A ; also compare with area A in 

 photographic figure, plate 4, figure 19. Life size. Page 173. 



Figure 22. — The same specimen; showing the ventral termination of 3 interam- 

 bulacra. A, C and /, and 2 ambulacra, B and /. The latter have 

 4 columns of plates ventrally, like Melonites multiporus (plate 2, figure 

 2). The dotted lines at the ventral border of area .4 indicate a resto- 

 ration of the 2 plates which are wanting in all 5 interambulacra. 

 Magnified 2 diameters. Compare with same areas in photographic 

 figure, plate 4, figure 19. ^Magnified 2 diameters. Page 174. 



Figure 23. — The same specimen; interambulacral plate with spine bosses. Com- 

 pare with photographic figure, plate 4, figure 19. Magnified 4 diam- 

 eters. Page 174. 



Figure 24. — The same specimen ; ambulacral detail, taken from right hand lower 

 side of area B at the point X (see photographic figure). Only 5 col- 

 umns of ambulacral plates are seen in this figure, whereas 6 occur 

 higher up on the corona, in each half ambulacrum, as at F(plate 4, 

 figure 19). This area was figured, however, as showing detail the 

 clearest of any portion for figuring. Magnified 2 diameters. Fig- 

 ures 21-24 are from the same specimen as plate 4, figure 19. Page 174. 



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