116 Prof. W. Thomson on the “ Vitreous” Sponges. 
among their meshes (Diplodemia &c.); so that I believe we 
may regard the whole group as potentially siliceous. | 
Dr. Schmidt’s second family are the Ceraospongie :—‘‘ Spon- 
giz quarum sceletum formatur fibris solidioribus, recenti statu 
plus minusve elasticis, quee seepius aliena corpuscula involvunt, 
sed nunquam spicula in ipsis nata continent.” 
His third family are the Gumminee :—‘ Spongiz Corneo- 
spongiis proxime. Parenchyma spississimum et maxime com- 
pactum, adspectu Kautschuk, quod tamen fibrillis tenuissimis 
contextum est. Generum pars corpuscula silicea continent.” 
The fifth family in Dr. Schmidt’s arrangement are the 
Halichondria :—‘‘ Spongie spiculis siliceis pertextee, quae ob 
telam laxiorem et minus spissam quamquam szpius subcor- 
neam neque Gummineis adnumerantur, neque Corticatis ob 
defectum strati corticalis.”’ 
The careful consideration of the diagnoses of these three 
groups is quite as suggestive as the examination of an extended 
series of the Sponges themselves, of a single graduated line of 
forms in which there are no breaks of sufficient importance to 
justify its subdivision into groups of higher value than fami- 
lies. 
The horn Sponges and the Gumminez are so nearly allied 
that they can be distinguished by comparative characters only. 
The fibres of the horn Sponges are thicker, and the meshes 
wider, the whole texture is more open than in the Gumminez, 
in which the minute fibres are matted together in the consistent 
sarcode, and the Sponge, when dried, looks like a piece of 
leather. The general aspects of the two groups are very dis- 
tinct; and even to the inexperienced eye the Gumminee form 
a natural and easily recognizable series, whose characters it is, 
however, scarcely possible to reduce to an intelligible defini- 
tion. From the absence of positive characters, it is evident 
that these two groups. are liable at any point to pass into one 
another. Among the Gumminee separate siliceous spicules 
appear in abundance, the fibrillation of the horny matter 
becomes obscure; and we thus pass almost imperceptibly into 
the fifth group, the Halichondrie. 
Professor Schmidt’s fourth group are the Corticate :— 
““Spongize globose vel tuberose; spiculis siliceis pertexte, 
peer strato corticali cireumdate, quod et tela organica 
rmiori fibrillosa et plerumque corpusculorum siliceorum ge- 
nere a parenchymate interiori differt.”’ 
In this group we have positive characters of some value in 
the very marked difference between the cortical layer and the 
central mass, in the regular arrangement of the various histio- 
logical elements, and in the peculiar type of the defensive 
