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Show 70 DR. J. S. BOWEKBANK ON SILICEO-FIBROUS SPONGES. [Jan. 28, work," it will include six other genera, Halichondria, Hyalonema, Isodictya, Spongil/a, Diplodemia, and Desmacidon. W e have thus no less than eighteen genera, not one of which has a particle of true siliceo-fibrous structure in their skeletons, incorporated with Dactylocalyx 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 knowledge 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 established canons of nomenclature, proposed to abrogate the established generic names of the working naturalists who have preceded him id writing ou 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 professor'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 Spongiadae ? In the extensive 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. In all the genera comprised in the siliceo-reticulate and spiculo-fibrous sponges nature has provided in their structure for their capability of expanding and contracting their skeletons to a certain limited extent; and this power appears to be inherent in all parts of the animal mass. W e therefore find the dermal integuments closely adherent to the surface of the animal, expanding and contracting in unison with the general mass. This is not the case with a 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 |
