Explanation of Plate 6, 



^ Figure 25. — Ollgoporus coreyi, M. and W. ; viewed from the inside, so areas must be 

 reversed for comparison with outside. Interambulacra have 2 plates 

 ventrally, as in Melonites, plate 2, figure 3. In the right hand area 

 plate 1 is broken ; plate 2 is entire and has an angle, like plate 3, 

 figure 10, for reception of single initial plate. Columns 3, 4, 5 are 

 introduced as in Melonites. The ambulacra ventrally have 2 plates, 

 a b; dorsally these pass into 4 plates, a a^ and 6^ b. This explains 

 the relations of the several columns of ambulacral plates in Oligoporus, 

 figure 30, and Melonites, plate 2, figures 2, 4, to the 2 columns char- 

 acteristic of most Echmi, plate 8, figures 43, 47, 48. (See diagram on 

 page 191.) Subcarboniferous, Indiana. Specimen in Museum of 

 Comparative Zoology, catalogue number 3008. Magnified 3 diam- 

 eters. Pages 142, 189. 



Figure 26.-'— The same ; restoration of ventral border of right interambulacrum of 

 figure 25, showing dorsal border of the supposed initial plate V in 

 place ; restored portions are shaded. Page 192. 



Figure 27. — The same; farther restoration, showing the probable form of plates 1 

 and 2 and initial plate V before resorption had taken place ; compare 

 with plate 3, figures 8-11, and plate 7, figure 42. Page 192. 



Figure 28. — The same; another interambulacrum in same slab and probably the 

 same individual as figure 25 ; viewed from the inside, showing ar- 

 rangement of plates. Life size. Pages 186, 193. 



Figure 29. —The same; interambulacral plate, with spine bosses. Magnified 1.5 

 diameters. Page 187. 



Figure 30. — Oligoporus danse, M. and W. ; ambulacral area. The columns a, a\ y, b 

 compare with 2 columns, a, b, seen ventrallj'' in figure 25. Isolated 

 plates occur in the middle of each half ambulacrum ; these appear 

 to be the equivalent of plates c, d in Melonites, plate 2, figure 4. 

 Keokuk group, Subcarboniferous, Keokuk, Iowa. Specimen in Mu- 

 suem of Comparative Zoology, catalogue number 2998. Magnified 2 

 diameters. Page 190. 



Figure 31. — Oligoporus danse ; showing arrangement and development of interambu- 

 lacrum. A break occurs in column 9, when after 2 intervening rows 

 are built, plates are again added to column 9. Considerable irregu- 

 larity of plates occurs at this area, as described. Keokuk group, Sub- 

 carboniferous, Warsaw, Illinois. Specimen in Museum of Comjmra- 

 tive Zoology, catalogue number 2997. Life size. Page 193. 



Figure 32. — Oligoporus danse ; spines. Keokuk group, Subcarboniferous, Alton, Illi- 

 nois. Specimen in Yale University Museum. Magnified 6 diam- 

 eters. Page 196. 



Figure 33. — Oligoporus danse ; showing enlargement of pores and spaces between 

 plates by silicification. Keokuk group, Subcarboniferous, Alton, 

 Illinois. Specimen in Yale University Museum. Magnified 4 diam- 

 eters. Page 197. 



Figure ^4..— Oligoporus danse'; showing arrangement and development of area, after 

 Meek and Worthen (30). Modified from their figure by correcting 

 orientation, omitting ambulacra and spine bosses, and introducing 

 numbers and dotted lines to accentuate columns. Keokuk group, 

 Subcarboniferous. Page 197. 



Figure 35. — Oligoporus nobilis, M. and W. ; plates showing spine bosses. Keokuk 

 group, Subcarboniferous, Keokuk or Warsaw, Illinois. Specimen 

 in American Museum of Natural History. Magnified 1.5 diameters. 

 Page 198. 



251 



XXIX— Bull. Geol. Soc. Am., Vol. 7, 1895. 



