536 NORTH AMERICAN INDEX FOSSILS. 



separated from one another by supplementary pieces. Posterior 

 oral highly convex and about as large as the other four together; 

 from it a ridge extends to the bottom of the calyx and is inflated 

 around the anal opening. Arms 12. 

 Keokuk of Indiana. 



XLIII. Periechocrinus Austin. 

 Calyx large, elongate, bell or urn-shaped. Plates thin, smooth 

 or delicately sculptured, the R marked generally with a ridge which 



passes from plate to plate 

 and increases in promi- 

 nence upward till it be- 

 comes identified with the 

 free arms. B 3, equal, 

 forming a deep cup. R 

 and costals long and nar- 

 row and constricted above 

 and below. Costals 2, six- 

 or seven-angled, followed 

 by two to four rows of 

 distichals and usually two 

 „ . , . ,AA AA7 11 ^ ^o six palmars. Arms 



Periechocrinus. (After Weller.) 



branching, long and slen- 

 der. Ventral disk moderately convex or almost flat, composed 

 of small and irregularly arranged plates. Anus nearly central. 

 Column large and cylindrical. (Fig. 1864.) Siluric-Mississippic. 



119. P. christyi (Hall). (P. zvhitfieldi Hall.) Siluric. 

 Resembles P. tennesseensis in outline, but larger, and surface 



marked with excentric lines of fine granules, parallel to the mar- 

 gins of the plates. 

 Niagaran of Indiana. 



120. P. chicagoensis Weller. Siluric. 

 Calyx small, constricted below and deeply depressed between the 



arm bases, which stand out conspicuously. Arms 10 (two from 

 each ray). 



Niagaran of Illinois. 



121. P. marcouanus (Winchell and Marcy). Siluric. 

 Very large and elongate (sometimes three inches long by nearly 



