5/6 NORTH AMERICAN INDEX FOSSILS. 



Group of Mississippi. Field Museum of Natural History publication, 



no. 134; Geol. Series, Vol. IV., no. i. 



Specific descriptions are also scattered through the writings of Clark 

 (Md. Survey); Conrad; Gabb (Pal. Cal.); Hall (Iowa Geol. Surv., 

 vol. I, pt. 2); Keyes (Geol. Mo., IV.); McCrady (Plioc. Foss. S. 

 Car.); Meek and Meek and ^Vorthen (Geol. Surv. 111., vols. 2, 5, 

 7); Morton ; Ravenel (Journ. Acad. Nat. Sci., VIII., and Cat. of the 

 Echinoidea — Rec. and Foss. — S. Carol.); Remond (Proc. Cal. Acad. 

 Sci., ser. i. III.); Say ; \A^eller (Pal. N. J., IV.), and others. The 

 most recent and important work on the classification of Echinoidea is P. 

 M. Duncan, 1889, Revision of the Genera and Great Groups of the 

 Echinoidea. Journ. of the Linnean Society (Zoology) Vol. XXIII. 



Key to the Genera. 



A. Palceechinoidea. Test of more than twenty meridional columns of plates, each 



interambulacrum having more than two columns, or less than twenty [Bothrio- 



cidaris) when ambulacra are represented by one column only in each zone I. 



I. Interambulacral plates large, each with a large single spine boss in center i. 



I. Ambulacra of two columns ; interambulacra of 4-7 ; peristome large a. 



a. Tubercles of interambulacra surrounded by ring of granules. 



II. Archceocidaris. 



a. Tubercles not surrounded by ring I. Eocidaris. 



1. Ambulacra of 2 -j- columns ; interambulacra of 8 columns. 



III. Lepidocidaris. 



I. Interambulacral plates without large spine boss 2. 



2. Ambulacra of two columns of plates b. 



b. Plates strongly imbricating IV. Lepidechinus. 



b. Plates not imbricating V. Rhoechinus. 



2. Ambulacra of more than two columns c^ 



c. Plates not imbricated *. 



*. Ambulacra 2 -|- columns ; interambulacra 5-7 co\\xvsys\%....Paluechinus. 

 *. Ambulacra with 4 columns ; interambulacra 4-9 columns. 



VI. Oligoporus. 

 *. Ambulacra 6-14 columns ; interambulacra 4-H columns. 



VII. Melonites. 



c. Plates of corona imbricated **. 



**. Plates regular VIII. Lepidesthes. 



**. Plates irregular IX. Fholtdocidaris . 



B. Euechinoidea. Test of twenty columns of plates, 2 in each ambulacrum and 2 



in each interambulacrum (except Tetracidaris which has 4 in each interambu- 

 lacrum) II. 



II. Test regular ; with peristome and anal opening (periproct) at opposite poles..3. 



3. Cidaroida. With discontinuous perignathic girdle ; both ambulacral and 



interambulacral plates continued beyond peristome to the true mouth. 

 Ambulacral plates typically simple, each with one pair of pores d. 



d. Ambulacral areas narrow, undulating ; pores close together ; primary 

 tubercles perforated and crenulated, spines cylindrical to spindle-shaped, 

 generally granulose or spinulose X. Cidaris. 



d. Ambulacral areas straight or undulating ; primary tubercles not crenij- 

 lated ; spines large, smooth, cylindrical. XI, Leiocidaris. 



