446 Scientific Proceedings, Royal Dublin Society. 



EXPLANATION OF PLATE XXI. 



Figs. 1 to 3 — Illustrations of Graticulmia calathus. 



Fig. 1. — Hollow casts of fragments of hexactinellid 

 network, seen by reflected light. Obj. 1 

 inch ; oc. No. 2. 



,, 2. — The sponge. Nat. size. 



,, 3. — One half of the sponge, showing the oscules 

 of the interior. 



Figs. 4 to 5 — Platychonia brodiei. 



Fig. 4. — Section through the sponge, showing the 

 skeletal network. 



,, 5. — One half of the sponge, showing a longitu- 

 dinal median section. The highly shaded 

 portion indicates the matrix, which partly 

 fills up the interior of the sponge. 



,, 6. — The sponge. Nat. size. 



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