582 



NORTH AMERICAN INDEX FOSSILS. 



IV. Lepidechinus Hall. 

 Plates of corona of type of Archcuocidaris, but strongly imbri- 

 cating; ambulacra with two columns imbricating adorally (down- 

 wards) ; interambulacra with 8-11 columns imbricating aborally 

 (upwards) and from center outwards. Devonic-Mississippic. 



9. L. rarispinum Hall. Mississippic. 

 Interambulacral areas with 11 columns of plates in adult (am- 

 bitus), where they are elevated into short spines. 



Waverly of Ohio and Pennsylvania. 



V. Rhoechinus Keeping. 

 Plates of corona not imbricating, without large tubercles, but 

 covered by numerous pits for insertion of spines ; ambulacra of two 

 columns, interambulacra of 4-8. Mississippic-Carbonic. 



10. R. burlingtonensis Meek and Worthen. Mississippic. 

 Six columns of interambulacra in adult. 



Burlington of Iowa. 



11. R. gracilis Meek and Worthen. (Fig. 1915, ^.) Mississippic. 

 Eight columns of interambulacra in adult. 



Burlington of Iowa and Ken- 

 tucky. 



VI. Oligoporus Meek and 

 Worthen. 



In general form and propor- 

 tions like Melonites, but the 

 ambulacra have only four col- 

 umns of plates and the inter- 

 ambulacra five to nine at equa- 

 tor or ambitus (adult charac- 

 ter). Mississippic. 



12. 0. nobilis Meek and 

 Worthen. (Fig. 1916.) 



Mississippic. 



With four columns of interambulacral plates in the adult. 

 Burlington of lUinois ; Keokuk of Iowa. 

 13. 0. danae Meek and Worthen. (Fig. 1915,^-;.) Mississippic. 

 Four columns of ambulacral plates with occasional short plates, 



Fig. 1916. Oligoporus nobilis, X 

 (After Meek and Worthen.) 



