PLATE 50. 



Oligoporus danae (Meek and Worthen). 



Page 356. 



Fig. 1. Same specimen as photograph, Plate 49, fig. 4. X 0.8. Restored vcntrally as indicated by dotted lines; ambu- 

 lacrum at the mid-zone with occluded, demi-, and scattered isolated plates. Interambulaera each with nine columns 

 of plates, the ninth originating at or below the mid-zone. 



Fig. 2. Same specimen and view as the photograph, Plate 49, fig. 5. X 0.8. External view of interambulacrum E. The 

 adradial plates show lateral beveling where, when in place, they are inclined over by the ambulacral plates (compare 

 Plate 45, fig. 5). The ninth column originates in a tetragonal plate, the two wanting sides of which are compensated 

 for by two ventrally adjacent heptagonal plates. 



Fig. 3. Same specimen and view as the photograph, Plate 49, fig. 5. X 0.8. Internal view of interambulacrum I, restored 

 ventrally as indicated by dotted lines. Curiously the ninth column originates in a tetragonal plate with two adjacent 

 heptagons as in area E (fig. 2). It is rare for the initial plates of columns as introduced to be tetragonal but a similar 

 case is shown in the ninth column in Melonechinus, Plate 59, fig. 14, and in the fifth column in Lovenechinus, Plate 

 43, fig. 2. 



Fig. 4. Same specimen; left half-ambulacrum from the area marked X, in fig. 1. X 2.5. Demi-plates are wide, but the 

 occluded are wider, with scattered isolated plates. 



Fig. 5. Same specimen; right half-ambulacrum, from another area at the mid-zone. X 2.5. 



Fig. 6. Same specimen; genital plate with two oculars restored as indicated by dotted lines. X 2.5. The genital plate 

 has a strong bevel on its adoral face so that it is distinctly higher on the interior than it is on the exterior. 



Fig. 7. Keokuk Group, Lower Carboniferous, Keokuk, Iowa. Mus. Comp. Zool. Coll., 2,998. X 2.4. Ambulacrum 

 from the mid-zone, demi-plates are wide, but the occluded are wider, scattered isolated plates. (Compare text-fig. 

 12, p. 54; also compare figs. 1, 7, and 8 with text-fig. 237, p. 231.) 



Fig. 8. Keokuk Group, Lower Carboniferous. Washington County, Indiana. F. Braun Coll. X 2.5. Ventrally ambula- 

 cral plates are primaries only; higher up a few plates are primary and occluded; higher still demi- and occluded, but 

 as yet no isolated plates have appeared. In the interambulacra there are two plates in the basicoronal row, three 

 plates in the second row, and four plates in the third row. the fifth column originating in the fifth row; a few tubercles 

 and spines are shown in area A. 



Fig. 9. Same specimen. X 2.5. Left half-ambulacrum from a higher zone, showing the typical generic character with 

 isolated plates in the middle of the half-area. 



Fig. 10. Keokuk Group, Lower Carboniferous, Jersey County, Illinois. F. Springer Coll., 8,124. X 3.3. Interambula- 

 cral plate with secondary tubercles. 



Fig. 11. Lower Carboniferous, Missouri. Mus. Comp. Zool. Coll., 3,146 (from R. T. J. Coll.). X 2.5. Pyramid in side 

 view, with ridges for the attachment of interpyramidal muscles. 



Fig. 12. Same slab; pyramid. X 2.5. An internal view, showing the dental slide, pyramidal suture, and an ppiphysis 

 in place. 



