65 



PLATE XXV. 



MIOROCIONA AMBIGUA, Bmoerlcuil: 



Vol. ii, p. 136, ' Mon. Brit. Sponges.' 



Fig. 1. Represents the type-specimen on a portion 

 of the shell of Pinna ingens ? Natural size. 



The sponge is so thin that, although it extends in 

 one unbroken sheet over the surface of the shell, it 

 does not conceal the small parasitic shells beneath it 

 which were previously deposited on the large one. 



Fig. 2. One of the attenuato-acuate basally-spined, 

 skeleton spicula. X 80 linear. 



Fig. 3. The basal portion of one of the skeleton 

 spicula. X 250 linear, to show the minuteness of their 

 spination. 



Fig. 4. One of the slender acerate, tension spicula 

 of the dermal membrane. X 250 linear. 



Fig. 5. One of the attenuato-acuate, entirely spined, 

 internal defensive spicula. X 250 linear. 



Fig. 6. One of the sub-attenuated, cylindrical, 

 entirely-spined, tension spicula of the interstitial mem- 

 branes. 



The forms of the spicula are various and frequently 

 much distorted. X 250 linear. 



Fig. 7. One of the angulated, bidentate-equian- 

 chorate, retentive spicula. X 530 linear. 



Fig. 8. One of the bidentate, palmate, equianchorate, 

 retentive spicula. X 530 linear. 



Fig. 9. A section of the sponge at right angles to 

 its surface. X 80 linear. 



The arched appearance of the basal outline of the 

 section is caused by the projecting ridges of the shell 

 on which it is seated. 



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