PLATE IV. 



ISODICTYA SCITULA, BowerbanJc. 



Figs, la, Ib, Ic. Acuate spicules of the skeleton, 

 to show their variation in size. X 150. 



Fig. 2. One of the bidentate, equi-anchorate, reten- 

 tive spicula. X 800. 



Figs. 3 and 3&. Angulated bihamate spicula of the 

 dermal membrane, more abundant than the last but 

 not mentioned in the description of the species. 

 X 800. 



For figures of the sponge itself see Plate IX, figs. 

 13. 



HYMENIACIDON VIRGULATUS, Bowerbank. 



Fig. 4. The sponge. Natural size. 

 Fig. 5. One of the long, slender, acuate spicula of 

 the skeleton. X 150. 



HYMENIACIDON CALLOSUS, Boiverbank. 



Figs. 6 and 7. The type-specimens of the sponge. 

 Natural size. 



Fig. 8. One of the acuate spicula of the skeleton. 

 X 150. 



[Fig. 9. A hydroid zoophyte, parasitical on H. cal- 

 losus, and inserted in the Plate by the engraver by 

 mistake.] 



HYMENIACIDON ARMIGEE, Bowerbank. 



Fig. 10. Hymeniacidon armiger. Natural size. 

 Thinly coating the basal portion of a specimen of 

 Nullipora polymorpha. 



Figs. 11, 12 and 13. Attenuato-acuate, entirely 

 spiiied, dermal spicules, showing their variability in 

 size. X 150. 



Fig. 14. A fusiformi-acerate spicule of the skele- 

 ton. X 150. 



Fig. 15. A bidentate equi-anchorate spicule of the 

 dermal membrane. X 800. 



Figs. 16 and 17. Tridentate, equi-anchorate, reten- 

 tive spicula. X 800. 



