Fig. 35. Strongylocentriis drcebakensis O. F. M. Young specimen, of 1,2 

 mm., from spirits in Canada balsam, veutral half seen from the 

 outside. The primary ambulacral plates I. V, IV b, drawu as if 

 transparent. The peristome beginning to be eroded, the solitai'y 

 first plates of the interradia remaining for a great part intact. The 

 primary ambulacrals still close to their columns. 



Fig. 26. The same specimen, ventral portion, seen from within. The au- 

 ricles beginning to form under the retractor muscles. 



Fig. 27. The same specimen, dorsal lialf viewed from the outside. The dorso- 

 central disk still for a great part retains its original pentagonal 

 shape. 



Fig. 28. Another specimen, 2,2 mm. in diameter. Tlie mouth opened. The 

 primary ambulacrals, heterotropic, still close to their columns; the 

 peristome slightly more eroded than in fig. 25, a lesser part of the 

 solitary first interradials remaining; the gills have appeared. 



