Plate 23. 



Microiihotograplis of thick unstained preparations showing skeletal arrangement. 

 Fig. 1. Tijl(i<lesina vcstUnihiris. Section vertical to surface of sponge, which is repre- 

 sented by right margin of figure. Hexactinellid 

 skeleton appearing in deeper part of section ; x 25. 



" 2. " " Dermal membrane covering in a large vestibular cavity. 



Upper right margin represents edge of osculum ; 

 X 15. 



" 3. " " Dermal membrane covering in, and adjoining, a medium- 



sized vestibular cavity ; x 15. 



" 4. lojihon indentatus. Section vertical to surface, which is represented by right 



margin of figure. Section includes large part of the 

 Gortjonia axis (to the left) ; x 25. 



