Explanation of Plate 7. 



"Figure 36. — Rhoechinus gracilis, iNI. and W. ; sandstone cast, seen from the inside, 

 so that areas indicated by numbers and letters must be reversed for 

 comparison -with the ontside. The specimen sliows more or less 

 completely the arrangement and development of plates in five inter- 

 ambulacral areas. The amlnilacral areas are clearly seen, but the 

 individual plates are not visible in the specimen; they are restored 

 in area / as indicated by dotted lines. Genital and ocular plates are 

 visible dorsally. Waverly group, Subcarboniferous, Menifer county 

 Kentucky. Specimen in Student Collection, Harvard University. 

 Catalogue number 115. Magnified 2 diameters. Page 201. 



* FiGURii: 37. — Rhoechinus gracilis; ambulacral detail. Burlington group, Subcarbonif- 

 erous, Burlington, Iowa. After Meek and AVorthcn (31). Magni- 

 fied 2 diameters. Page 202. 



- FiGURi: 38. — Palxechinus gigas, M'Coy; interambulacrum showing plate arrange- 

 ment. The right column, 2, of adam])ulacral ])lates is restored, as 

 indicated by dotted lines. Carboniferous, Clitheroe, Lancashire, 

 England. Slightly enlarged. Page 204. 

 Figure 39. — Palnrchinus gigas ; detail of one half-ambulacrum. Carboniferous, 

 Clitheroe, Lancashire, England. Figures 37 and 38 from specimen 

 in the Museum of Practical Geology, Jermyn street, London ; com- 

 pare with figure 1, page 191. Magnified 1.5 diameters. Page 205. 

 ' FiGURi: 40. — liJioecJiiinifi elegavs, ^V Coy ; details of ambulacrum. This should be 

 compared with the amljulacrum at ventral border of area in Ollgo- 

 poriis, plate 6, figure 25; also figure 1, page 191. Carboniferous, 

 Hook Head, Ireland. Si)ecimen in jNIuseum of Comparative Zo- 

 (')logy, catalogue number 3002. ^Magnified 4 diameters. Page 205. 



^ Figure 41. — Lepidocidaris squamosus, M. and W. ; showing arrangement of plates 

 and introduction of the sixth, seventh and eighth columns. Com- 

 pare with Melonites, i)late 5, figure 20, and Archivocidaris, plate 8, 

 figures 44, 45. The ambulacra consist of 2 columns ventrally, but 

 higher up pass into 4 imperfect columns. Compare with OligoporuS 

 rorciii, ]>late (i, figure 25. Burlington group, Subcarboniferous, Bur- 

 lington, Iowa. Specimen in Museum of Comparative Z<)()logy, cata- 

 logue numl)er 3026. This .specimen is the type of the genus and 

 species. Life size. Pa<;e 220. 



> FiGURic 42. — Lt'pidecJiinus rarispinus, Hall ; showing a single initial plate, 1^, ven- 

 trally, succeeded by 2 plates, 1, 2; (compare with plate 3, figures 

 8-11, plate 6, figures 24-2(5, plate 9, figure 54; also figures 3 and 4, 

 page 234.) iNIore columns, from 3 to 9, inclusive, are added dor- 

 sally, as indicated by numbers and dotted lines. The plates on 

 account of their imbrication are not so regular and are different in 

 form from Melonites and Oligoporus ; they also differ from these in the 

 rapid or accelerated rate of introduction of colunms. Waverly group, 

 Warren, Pennsylvania. Specimen in the collection of Professor 

 C. E. Beecher, of New Haven, Connecticut. Magnified 2 diameters. 

 Page 226. 



252 



