5o6 NORTH AMERICAN INDEX FOSSILS. 



Surface divided by strong ridges into deep triangular spaces. Ven- 

 tral sac more than five times the length of the calyx. 

 Cincinnatian of Ohio. 



XII. BoTRYOCRiNUS Angcliu. 



Dorsal cup obconical, with truncated base. IB 5, somewhat pro- 

 tuberant. B 5. R large, with concave facets for the arms. Ven- 

 tral sac one half to two thirds the length of the arms, supported by 

 the small radianal plate recurved toward the summit. Arms uni- 

 serial, 10. Anus anterior, at base of the ventral sac. Siluric. 

 2^. B. polyxo Hall. Siluric. 



Plates of the dorsal cup usually slightly prominent in the center, 

 with low, angular ridges extending to the sutures. 



Niagaran of Indiana and Illinois. 



XIII. Cyathocrinus Miller (emend. Wachsmuth and Springer). 

 Dorsal cup bilaterally symmetrical, globose, cup-shaped, with 

 convex sides incurving above. IB 5, equal. B large, the posterior 

 B truncated for the support of an anal plate. R 5, all simple, their 

 upper faces provided with a facet occupying less than the full 

 width of the plates. Anal plate one between the R ; its succeeding 

 plates not enclosed in the calyx, but forming part of the ventral 

 sac. Ventral sac rarely extending more than half the height of the 

 arms. Number of costals extremely variable among the rays. 

 Arms rather deHcate, composed of elongate cylindrical joints and 

 giving off numerous branches most of which divide again. Stem 

 round. Ordovicic-Mississippic. 



24. C. cora Hall. Siluric. 

 Dorsal cup very large (one to one and one half inches in diam- 

 eter), subglobular, usually found as internal molds. Surface cov- 

 ered with groups of parallel ridges which cross the sutures at right 

 angles. R constricted at the arm bases, forming five rounded 

 interradial protuberances. Arms very slender, dividing at once 

 into two main divisions which extend out horizontally and give off 

 vertical branches on the upper side. 



Niagaran of Illinois and Wisconsin. 



25. C. enormis Meek and Worth en. Mississippic. 

 Like C. iowensis, but thinner and more conical. 



Lower Burlington of Missouri and Iowa. 



