PLATE XXXI. 



PAGE. 

 Fig. 1. AKCH.SOCIDAEIS ILLINOIENSIS 338 



1 a, 6. Primary spines natural size. 



Fig. 2. COMPSASTER FORMOSUS 327 



2 a. Ventral side natural size. 



2 &. Section of ray enlarged two diameters. 



Fig. 3. TREMATASTER DIFFICILIS 330 



3 a. Ventral side magnified two diameters. 



3 b. Section of ray magnified four diameters. 



Fie, 4. CHOLASTER PECULIAHTS 32i> 



4 a, Dorsal view natural size. 



4 6. Section of ray magnified two diameters. 



Fig. 5. HYBOCHINDS SPECTABILIS 332 



5 a. View of a depressed specimen natural size. 



5 b. d. Views of ambulacral and adambulacral plates, enlarged two diameters. 



5 c. Magnified view of horn-like protuberance. 



Fig. 6. View of a crushed specimen, showing the jaws. 



Fig. 7. JAWS, belonging to this or a kindred species. 



Fig. 8. PERISCHODOMUS ILLINOIENSIS 33S 



View of the apical end of a distorted specimen, natural size. 



Fig. 9. ECHINODISCUS OPTATUS 336 



View of part of the lower side of a specimen, showing the plates near 

 the margin, natural size. 



