Plate 22. 



Miciophotographs of thick unstained preparations showing skeletal arrangement. 

 Fig. 1. Gellkis perforatus. Section vertical to surface, which is represented by right 



margin of figure ; x 15. 

 " 2. Ti/lodesma alba. Section vertical to surface, which is represented by right 



margin of figure. Dermal surface in upper part of figure 

 is roughened with projecting spicular tufts, elsewhere 

 smooth ; x 15. 



" 3. " " Section vertical to surface, which is represented by right 



margin of figure. Surface is smooth, although it scarcely 

 appears so, owing to the fact that it is partially seen in 

 perspective ; x 15. 

 " 4. Tylodcsma vestibularis. Section vertical to surface, which is represented by right 



margin of figure. A vestibular space is cut across. 

 The hexactinellid skeleton is conspicuous in the deeper 

 part of the section ; x 15. 



