B. The silicate skeleton of the PHÆODARIA was formerly taken by me for a purely siliceous one. When I described the first PHÆODARIA in my Monograph in 1862, I was only acquainted with five genera and seven species, whilst the number of PHÆODARIA here described from the Challenger amounts to eighty-four genera and four hundred and sixty-five species. In the vast majority of these (though not in all) the skeleton becomes more or less intensely stained by carmine, and is also more or less charred at a red heat, in some even becoming of a blackish-brown. In many PHÆODARIA, furthermore, the hollow skeletal tubes are destroyed by the continued action of heat. They are also, for the most part, strongly acted upon, or even destroyed by boiling caustic alkalis, whilst boiling mineral acids have no effect upon them. The best method of cleaning the skeletons of PHÆODARIA from their soft parts is to heat them in concentrated sulphuric acid, and then add a drop of fuming nitric acid; in this they are not dissolved even on prolonged heating. From these facts it would appear that the skeletons of the PHÆODARIA consist of a compound of organic substance and silica, or a "carbonic silicate." The more intimate composition yet remains to be discovered, as also the manifold differences which the various families of PHÆODARIA seem to show in respect of its composition. The small skeletal fragments of the Dictyochida (the only remains of PHÆODARIA which occur as fossils) appear to consist of pure silica.