584 



NORTH AMERICAN INDEX FOSSILS. 



bulacra beginning as four, but subsequently reduced to three 

 columns. 



Keokuk of Indiana. 



Fig. 191 7. 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 full 

 number of columns is shown by plates along line, X- V. a, genital plates, with 3 or 4 

 pores; 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.; D.D, 

 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 columns. 

 Keokuk of Indiana. 



