340 PROCEEDINGS OF THE ACADEMY OF [1890. 



Fig. 17. Ventral surface. 



Fig. 18. A portion of the ventral surface, enlarged two diameters to 



show the three rows of ambulacral spines. 

 Fig. 19. Thyraster serpentarius. Portion of the dorsal skeleton of 



an arm, enlarged about four diameters. 

 Fig. 20. Portion of the ventral skeleton of same. 

 Fiff. 21. Portion of the lateral skeleton of same. 



