North American index fossils, invertebrates . g ven-trally. A ventrally adjacent plate of one of the bounding columnsis characteristically seven-sided. Newly added plates dorsally aremore or less rhombic in Outline. This facilitates the orientationof obscure fragments. Mississippic. 14. M. multiporus Norwood and Owen. (Fig. 1917, a-c.) Mississippic. With ten ambulacral and seven to eight or nine interambulacralcolumns of plates. The most characteristic fossil of the St. Louis limestone ofMissouri, Indiana, Tennessee, Kentucky, etc. VIII. Lepidesthes Meek and ; plates of corona reg
North American index fossils, invertebrates . g ven-trally. A ventrally adjacent plate of one of the bounding columnsis characteristically seven-sided. Newly added plates dorsally aremore or less rhombic in Outline. This facilitates the orientationof obscure fragments. Mississippic. 14. M. multiporus Norwood and Owen. (Fig. 1917, a-c.) Mississippic. With ten ambulacral and seven to eight or nine interambulacralcolumns of plates. The most characteristic fossil of the St. Louis limestone ofMissouri, Indiana, Tennessee, Kentucky, etc. VIII. Lepidesthes Meek and ; plates of corona regular and imbricated; interambulacrafrom 3-7 columns, imbricating aborally and laterally; ambulacrabroad, from 8-18 (or perhaps 20?) columns, imbricating adorallywith pores in center of plates. Mississippic. 15. L. wortheni Jackson. (Fig. 1917, J.) seven to eight columns of ambulacra in adult and interam- 584 NORTH AMERICAN INDEX FOSSILS. bulacra beginning as four, but subsequently reduced to threecolumns. Keokuk of Fig. 1917. a-c, Melonites muliiporus, a part of corona from oral or ventral end,showing two columns at peristome (^, C and /) and three ^E, G). Amb. 4. If,dorsal portion of interambulacrum, and the separation of plates in columns. The fullnumber of columns is shown by plates along line, X- V. a, genital plates, with 3 or 4pores; 0, imperforate ocular plates, X ^/^ > ^t spines, X 4 > ^> Lepidesthes wortheni,a crushed specimen, X I/^/showing broad amb. area and 4 (later 3) interamb.; ,dental pyramids ; e, Lepidechinus rarispinus, oral aspect, showing single interamb. in-creasing to 8, X iK- (After R. T. Jackson and T. A. Jaggar, Jr ) 16. L. colletti White. Mississippic. With 18 (or 20?) ambulacral, and 4-5 interambulacral of Indiana. ECHINODERMA TA—ECHINOIDEA. 585 IX. Pholidocidaris Meek and Worthen. With coarse, highly irregular, imbricating interambulacral plates,of which it has five or more columns; and six or
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