758 
DE. J. S. BOWEEBANK ON THE ANATOMY 
they are surrounded in a living state, the fibrous skeleton bears a striking resemblance 
to fibres of spun glass, and is quite as pellucid and colourless as the artificial material, and 
the dead sponge quite as brittle. The fibrous skeleton of Bactylocalyx pumicea^ Sttjtch- 
BUET, in its mode of arrangement strikingly resembles that of one of the sponges of com- 
merce ; it is equally complex and irregular in its structure, and the component fibres 
quite as much anastomosed. In that species the fibres are smooth and cylindrical, but 
in others they frequently abound with minute, obtuse, wart-like elevations. 
There is every indication in the skeletons that the increase in diameter, and the exten- 
sion in length in the fibres are effected in the same manner as in the solid keratose 
fibres. The free terminations of the young fibres have the same attenuated but obtuse 
form, and the pullulation of the young fibres from the sides of the mature ones is quite 
as apparent as in their keratose congeners ; but, in the young state, they never appear 
to be viscid, as the keratose ones frequently are; and extraneous matters are never 
detected at their apices, or on their substance. 
There are two distinct forms of this class of fibre : — 
1st. Solid siliceous fibre. 
2nd. Simple fistulose siliceous fibre. 
The structure of solid siliceous fibre is very similar to that of solid keratose fibre. 
Occasionally there are indications of a former existence of a minute central canal, but 
in the fully developed fibre this is rarely visible. The external characters of these fibres 
vary in each species. In a new siliceous sponge in the British Museum, designated by 
Dr. Geay M°Andrewsia azdica, the fibres are quite smooth, as represented in Plate 
XXVIII. fig. 6. But in the greater number of species they are more or less tuberculated, 
as in Plate XXVIII. fig. 7, which represents a group of fibres from the type-specimen of 
Dactylocalyx jpumicea, Stutchbuey, a portion of which is in the possession of Dr. J. E. 
Geay. In other species in my possession the tuberculation is very strongly produced, 
as represented in a few fibres of Dactylocalyx Prattii, Boweebaatk, MS., Plate XXVIII. 
fig. 8. 
Of the 2nd form, simple fistulose siliceous fibre, I know but one example, and that is 
the remains of the siliceo-fibrous sponge on which the beautiful specimen of Eu^lectella 
cucumer^ Owen, is based. 
The tabulation of the skeleton-fibre is very similar to that of some varieties of simple 
fistulose keratose fibre, but the central cavities are not so invariably continuous as in the 
keratose varieties of fistulose skeleton-fibre. Fig. 11, Plate XXVIII. represents a small 
piece of the spinulated simple fistulose fibres of the skeleton of Dr. Aethue Faeee’s 
specimen. The spinulation of these fibres is a remarkable character. It is the only case 
of the production of acute spines on the skeleton-fibre of a siliceo-fibrous sponge with 
which I am acquainted. 
