Prof. W. Thomson on the “ Vitreous” Sponges. 115 
of Sponges from oe aes and memoirs in which external 
form is chiefly considered, such as the beautiful work of 
MM. Duchassaing and Michelotti, and Johnston’s ‘ British 
Sponges.’ 
The microscopic characters of Sponges, derived from the 
structure and form of the reticulated supports, and from the 
special forms of the spicules, are usually well marked; but 
these characters are in most cases of specific, in a few of ge- 
neric, and in scarcely any of ordinal value. Fortunately, the 
chemical composition of the organs of support and of defence, 
two or three well-defined general types of structure and form 
of the spicules, and the general arrangement and mode of 
combination of the different parts, present some good points of 
distinction for larger groups. 
I think that certainly the most satisfactory arrangement of 
Sponges is that proposed by Dr. Oscar Schmidt*. Dr.Schmidt’s 
memoir, however, labours under the disadvantage of dealing 
with the Sponges of the Adriatic only; so that many remark- 
able exotic forms, which might have modified to some extent 
his ideas of classification, are excluded. In the second Supple- 
ment, indeed, the author institutes a comparison between the 
Adriatic Sponges and those described by Dr. Bowerbank ; but 
the series is still incomplete, from the absence of illustration 
from the rich faune of the Kast and West Indies. 
_ Dr. Schmidt divides the Adriatic Sponges into six groups, of 
the rank of families}. The first of these are the Calcispongiz, 
which he thus defines :—“Spongie parve, plerumque albi- 
cantes, corpore spiculis calcareis pertexto.” 
It seems to be generally admitted that the Sponges with 
calcareous spicules are essentially distinct; and I am inclined 
to agree with Dr. Gray}, who places them in a distinct subclass, 
antithetic to the whole of the remainder of the Sponges, which 
form in his arrangement a second subclass under the name of 
PorIPHORA SILICEA. 
_ There is an evident awkwardness in placing such genera as 
_ Spongia and Halisarca (in which there is no silica whatever 
in any separate form) among the siliceous Sponges; still I 
think the classification is justifiable; and it is at all events 
convenient. ‘The true horn Sponges pass by almost imper- 
ceptible gradations into the groups which develope distinct 
siliceous elements, either within the fibres (e. g. Chalina) or 
* ‘Die Spongien des Adriatischen Meeres,’ Leipsic, 1862 (and two 
Supplements). 
f Handbuch der vergleichenden Anatomie. Jena, 1865. 
t “Notes on the Arrangement of Sponges,” Proc. Zool. Soc. Lond. 
May 9, 1867. 
