DR. WYVILLE THOMSON ON HOLTENIA. 
711 
The Zoological position and affinities of the Genus. 
In the ‘Annals and Magazine of Natural History’ for February 1868, I described a 
genus of siliceous sponges under the name of Habrodictyon , from specimens in the 
Museum of the Jardin des Plantes. The sponges which are now included in that genus 
were already well known ; but a general external resemblance and want of care in the 
discrimination of minute characters had previously caused a confusion between one of 
the species and JEuplectella aspergillum (Owen), the “ Venus’s flower-basket ” of the 
Philippines. While studying critically the Paris specimens with the view to the deter- 
mination of their affinities, I found to my surprise that a number of sponges from very 
different parts of the world, to which had only been attributed hitherto the common 
character of extreme beauty and rarity, were associated by structural and histological 
peculiarities sufficiently marked to distinguish them as a group from all the recognized 
orders of the Class Porifera. For these sponges I accordingly proposed to establish a 
new order, for which I suggested the name (Porifera) Vitrea. It is remarkable that 
nearly all the genera and species of sponges which we met with in the deep water of 
the warm area of the Atlantic were referable to the new order, and from their having 
been found living and in situ , the new forms threw a flood of light upon the relation and 
mode of occurrence of the group. 
In defining the position of the vitreous sponges, I have employed the classification 
proposed by Dr. Oscar Schmidt in his valuable work on the Sponges of the Adriatic *. 
I have, however, thought it necessary to propose certain slight modifications in his system, 
and the large number of additional genera which I have now had an opportunity of 
examining have led me to alter slightly my view of the sequence and affinities of the 
orders. The following outline of a classification of sponges, slightly modified from that 
of Dr. Oscar Schmidt, will serve to indicate the position which in the present state of 
information I should he inclined to assign to the vitreous sponges. 
Class Porifera, Grant. 
Subclass I. (Porifera) Calcarea, Bowerbank. “ Skeleton composed of calcareous 
spicules which are generally three-rayed stellate” (Gray), equivalent to Dr. O. Schmidt’s 
first family. Examples : Grantia, Sycon. 
Sub-class II. (Porifera) Silicea, Gray. “ Sponges provided with a siliceous or horny 
skeleton, or with a horny skeleton strengthened with siliceous spicules.” 
Order I. (P. Silicea) Vitrea, Wyville Thomson. Sarcode usually soft, containing 
but little formed horny matter in the form of minute granules. The skeleton consists 
entirely of siliceous spicules, either separate (in fascicles or scattered) or soldered together 
and combined into a continuous siliceous network. The sarcode contains small free 
spicules, different in character from the spicules of the skeleton, and frequently of com- 
plicated forms. In the typical sponges of the order all the spicules whether of the 
* Die Spongien des Adriatisehen Meeres. Leipzig, 1862 (and Supplements^. 
5 B 2 
