ECHINODERMA TA—CYSTOIDEA . 



461 



pierced by pores which are united in 

 pairs. Mouth nearly central. Minute 

 arms or spines spring from the ends 

 of the ambulacral grooves. Siluric. 



3. H. cylindricus Hall. (Fig. 1766.) 



Siluric. 

 Calyx ovate ; plates large and of 

 nearly equal size. (Type of genus.) 

 Niagaran of Wisconsin. 



4. H. alternatus Hall. (Fig. 1767.) 



Siluric. 



Calyx composed of twelve series of 

 plates alternating in size. Surface 

 strongly granulose. 



Niagaran of Wisconsin. 



ni. GOMPHOCYSTITES Hall. 



Elongate pear-shaped, composed of 

 many ranges of granulose, closely ap- 

 plied, polygonal plates. Openings fig. 1765. a, Eocystites hn. 

 upon outer surface, mouth subcentral gidadyius. (After Pack.) b, E- 

 and anus excentric. Arms sessile, P''^"'^''^'^ ^ pl^te enlarged. (Af- 



. . ter Walcott. ) 



lymg m grooves curvmg spirally out- 

 ward from the mouth to the greatest diameter of the calyx. 

 Siluric. 



Fig. 1766. Holocy^tites cylindricus, X 

 ^. (After Hall, 20th Mus. Rep.) 



Fig. 1767. Holocystites alternatus, X 

 :^. (After Hall, 20th Mus. Rep.) 



