120 BRITISH SPONGES: 



perforated mammillae ; but there are besides, in most specimens, 

 a few large orifices resembling the oscula of H. incrustans, being 

 level with the sm'face and having a ragged or substellated rim. 

 This unseemly sponge is not brittle and less friable than either 

 H. panicea or incrustans, to some varieties of which it has con- 

 siderable resemblance ; but it is always lighter and more fibrous, 

 and the surface, thickly studded over with its mammillated oscu- 

 la, is very characteristic. WTien the cuticular membrane is, from 

 any cause, destroyed, these are not of course apparent, but the 

 surface becomes then coarsely and loosely latticed, exhibiting in 

 fact the interior structure from which the parenchyma has been 

 washed out. Such an appearance H. panicea and H. incrustans 

 can never assume, for it is produced by the comparative tough- 

 ness, and indestructibility of the coarse fibres which are peculiar 

 to this species. These fibres may be compared to those of H. 

 fruticosa. They are tough, smooth, unequal in thickness, opake, 

 and composed of numerous spicula laid in parallelism. The 

 spicula are the same in shape as those which are found in equal 

 profusion in the intercellular parenchyma and in the cuticular 

 membrane, but where they lie crossed in every direction. They 

 are needle-shaped, — that is one extremity is rounded and obtuse, 

 when, after a short space, the shaft becomes a very little thick- 

 er, and is brought to a sharp point at the opposite extremity. 

 They are considerably longer than the spicula of H. incrustans, 

 and imlike those of H. panicea in shape, affording a certain test 

 by which we can distinguish it from them under every mask 

 it may assume. 



17. H. SABURRATA, o'ustaceous, the surface latticed tvith 

 large angulated open jwres ; oscula scattered, papillary ; spi- 

 cula short, curved and double-pointed. 



Plate XI. Fig. 3. 



Hub. Adheres to rocks in the Bav of Rouudstonc, Cun- 



