Prof. W. Thomson on the “ Vitreous’ Sponges. 125 
substance, composed mainly of a siliceous tubing formed es- 
sentially by the coalescence of spicules of the hexradiate type 
in every condition of distortion, is regularly spread, partially 
closing and reducing to rounded pores, by an oblique tissue of 
interlacing threads, the square meshes of the frame, and rising 
on the surface of the sponge into irregular spiral ridges. 
The mouth of the tube is closed, as in Habrodictyon corbicula, 
by a netted lid of dense tissue. Not taking into account dif- 
ferences in the form of the spicules, of specific value only—if 
the siliceous coil of Hyalonema were separated into small bun- 
dles and attached by transverse fibres within the wall of the 
tube of Habrodictyon, we should have a Sponge which would 
be referred without doubt to the genus Huplectella. 
Aphrocallistes is very nearly allied to Huplectella. There 
are the same fascicles of longitudinal fibres within the tube, and 
there is the same netted lid; but the tube is irregular in form, 
and the siliceous network is much more dense and compact. 
It will be remembered that some of the spicules of the sarcode 
in Aphrocallistes resemble those in Farrea, while others corre- 
spond with the form hitherto only known in Dactylocalyx*. 
In Dactylocalyx the longitudinal fascicles have disappeared, and 
the siliceous network is much more dense and irregular. 
specimen from Barbadoes, which I saw in the Paris Exhibition, 
had almost the cylindrical form of Aphrocallistes ; an example 
in the Belfast Museum is cup-shaped and looks like a silicified 
bath-sponge. I hope to have an opportunity of revising the 
whole of this genus or group of genera on some future occasion. 
The ultimate structure of its siliceous network and the close 
resemblance in form of its sarcode-spicules associate Dactylo- 
calyx clearly with Aphrocallistes and Farrea. | 
Farrea is undoubtedly a vitreous Sponge allied to Aphro- 
callistes. 'The hexradiate type of the framework, and the spi- 
cules figured by Dr. Bowerbank (Brit. Spong. figs. 199, 200), 
are very characteristic. [I am in doubt about the bihamate 
spicule (¢b¢d. fig. 114). From the condition of the only known 
example of Farrea occa, I should think it possible that one or 
many spicules of that form may have been mixed with it, 
possibly from some associated species of Hsperia. 
RELATIONS OF THE GROUP. 
It is difficult even to speculate upon the position of the 
vitreous Sponges in the series of the Porifera. There is 
_* Lam indebted to my friend Dr. Gray for an opportunity of examining 
the minute structure of Aphrocaliistes. ‘The woodcut represents a frag- 
ment of the general network of the wall of the tube, with some of the 
characteristic spicules of the sarcode alluded to above. 
Ann. & Mag. N. Hist. Ser. 4. Vol. i. 10 
