Explanation of Plate 3. 



Figure 8. — Strongylocentrotus drobachiensis, (O. F. M.); ventral border of part of 

 corona, showing primary ambulacral plates and succeeding ambu- 

 lacral plates of corona ; also the single initial plate 1^ of the inter- 

 ambnlacrum succeeded by 2 plates 1 and 2, in next row. Magni- 

 fied about 50 diameters. Kecent. Page 144. 



Figure 9. — Strongylocentyotas drobacJileasls ; later stage than figure 8, in which some 

 resorption of ventral border of corona has taken place. Figures 8 

 and 9 are portions of figures after Loven (27). Page 144. 



Figure 10. — Melon'des multiporus ; showing an angle on ventral border of plates 1 

 and 2 apparently for the initial plate V, which is restored in shad- 

 ing.* Cokimn 4 is introduced by tetragonal plate 4. Magnified 2 

 diameters. Specimen in Yale University Museum, diamond number 

 157. Pages 144 and 153. 



Figure 11. — Melon'des multiporns ; reconstruction, showing probable form of initial 

 plate V before resorption had taken place. This figure is adapted 

 from area A, plate 2, figure 2. Magnified 2 diameters. Page 145. 



Figure 12. — MelonUes multiporus ; interambulacrum in which the arrangement is 

 very clear from the size of the plates. Column 4 is peculiar in origi- 

 nating later than in any other specimen seen ; compare with photo- 

 graphic figure, plate 4, figure 18. Life size. Page 147. 



Figure 13. — Melondes multiporus ; showing dorsal portion of interambulacrum and 

 the rhombic form of newly introduced plates. The line A" Y, shows 

 that 8 columns exist in the dorsal area, although being strung out 

 they could not be counted in a horizontal i)lane at the same point 

 (compare figure 1, page 164) ; P, a pentagonal plate but not a terminal 

 of a column. Genital plates G have 3 or 4 pores ; ocular plates 

 are imperforate. Specimen in Johns Hopkins University. Magni- 

 fied 2 diameters. Pages 149, 155. 



Figure 14. — Melonites multiporus; showing variation. Columns 5, 6 and 7 are normal 

 in introduction, but column 8 originates in a hexagonal plate, 8, 

 which attains its extra side by making a reC'ntrant angle into the 

 adjacent octagonal plate 0. Specimen in Wagner Free Institute of 

 Philadelphia, accession number 3226. Life size. Page 151. 



Figure 15. — The same. Reconstruction of plate 8 with a pentagonal form, when 

 as a consequence the adjacent octagonal plate, see figure 14, becomes 

 a heptagon, //. Page 151. 



Figure 16. — Melonites multiporus ; showing variation. The initial plate of column 3 

 is pentagonal instead of hexagonal (compare figure 11). The second 

 plate, //, of column 3 is heptagonal. The initial plate of column 4 is 

 also heptagonal. Specimen in Yale University Museum, diamond 

 number 157. Magnified 2 diameters. Page 152. 



Figure 17. — Melonites multiporus; showing variation. There are two accessory pen- 

 tagons, P, P\ which are not terminal plates of newly added columns, 

 lying next the terminal pentagon of column 8, with its adjacent 

 heptagon. Specimen in Museum of Comparative Zoology, catalogue 

 number 2995. Magnified 1.5 diameters. Page 153. 

 All the Melondes figured on this plate are from the Saint Louis group, Subcar- 

 boniferous, Saint Louis, Missouri. 



248 



