70 DR.J.S. BOWERBANK ON S!LICEO-FIBROUS SPONGES. [Jan. 28, 
work,”’ it will include six other genera, Halichondria, Hyalonema, 
Isodictya, Spongilla, Diplodemia, and Desmacidon. We have thus 
no less than eighteen genera, not one of which has a particle of true 
siliceo-fibrous structure in their skeletons, incorporated with Dactylo- 
calyx and the other truly siliceo-fibrous species. Such a character, 
instead of facilitating the discrimination of species, is calculated to 
lead us unto a perfect maze of doubt and uncertainty ; and all this 
while he entirely ignores the existence of solid siliceous fibre. 
The author’s application of negative characters in his description 
of his order Vitrea is certainly bad: it is positive characters that 
lead us to correct discrimination of orders, genera, and species ; 
it is what they are that must be our guides, not what they are not. 
If Prof. Wyville Thomson had a more extensive and intimate know- 
ledge of the species of siliceo-fibrous sponges than he appears to 
possess, I can readily imagine that he would not have fallen into the 
errors that I have pointed out. 
Dr. Thomson, in his highly imaginative paper ‘‘ On the Vitreous 
Sponges,” has not only proposed a new and very impracticable 
order for their reception, but he has also, contrary to all the esta- 
blished canons of nomenclature, proposed to abrogate the established 
generic names of the working naturalists who have preceded him in 
writing on the siliceo-fibrous sponges; and, after criticising their 
differences of opinion very freely, he at once proposes that they 
shall all be abolished, and his newly concocted name Habrodictyon 
be established in their stead. If the new name were illustrative of 
new ideas, or of new facts, it might be entitled to consideration ; 
but as we find neither the one nor the other in the learned pro- 
fessor’s paper, I do not think he can reasonably expect that it will 
be adopted. 
Before we commence the descriptions of the genera and species of 
the siliceo-fibrous sponges, it will be as well to ask, what is a 
siliceo-fibrous sponge? and in what important points of structure 
does it differ from the general mass of the Spongiade? In the ex- 
tensive order Silicea we find by far the greater number of genera are 
characterized by the existence of siliceous spicula in their skeletons, 
and that they are separated from each other by peculiar modes of 
their arrangement in the structures. Inall the genera comprised in 
the siliceo-reticulate and spiculo-fibrous sponges nature has provided 
in their structure for their capability of expanding and contract- 
ing their skeletons to a certain limited extent; and this power 
appears to be inherent in all parts of the animal mass. We there- 
fore find the dermal integuments closely adherent to the surface of 
the animal, expanding and contracting in unison with the general 
mass. ‘Thisis not the case witha siliceo-fibrous sponge. The whole 
mass of the skeleton is formed of a continuous reticulation of solid 
siliceous fibre, which renders the skeleton perfectly inexpansible ; but 
to compensate for this apparent defect in its economy, these sponges 
are provided with a peculiar expansile dermal system, the dermal 
membrane being furnished abundantly with connecting spicula, the 
distal surfaces of which are closely cemented to the inner surface of 
