1869.] DR.J.S, BOWERBANK ON SILICEO-FIBROUS SPONGES. 69 
Dr. Wyville Thomson’s endeavour, by the institution of his pro- 
posed new order Vitrea, and his description of its characters, has, 
instead of elucidating the subject, still further complicated it. All 
the members of his new order should certainly agree in a series of 
definite structural characters; but this is not the case. Thus he 
gives, as examples of his order, Hyalonema and Dactylocalyz, the 
sponge of the first having a skeleton composed of spicula cemented 
together by keratode, as in the great mass of Halichondroid sponges, 
the mass of the skeleton being eminently elastic and entirely desti- 
tute of siliceo-fibrous structure, while in the latter genus the skeleton 
is perfectly rigid, being composed entirely of inelastic siliceo-fibrous 
tissue. 
But this is not the only error in the descriptions of the characters 
of his proposed new order Vitrea; thus he states (Ann. & Mag. Nat. 
Hist. p. 120) of the sarcode :—“ It is small in quantity, very soft, 
probably semifluid, extending in a thin layer over the fascicles of 
siliceous needles and over the siliceous framework.” 
Dr. Thomson’s description of the sarcode in this tribe of sponges 
is correctiy applicable as regards quantity, if, as appears to have 
been the case, he has derived his conclusions from an examination of 
the prepared skeletons of the sponges in the Museum of the Jardin 
des Plantes and the British Museum ; but it is a mistake to imagine 
that the sarcode is deficient in quantity when in their natural condi- 
tion. In Dactylocalyx heteroformis, D. M°Andrewia, D. Prattii, 
and D. Masoni which are in the same condition as when taken alive 
from the sea, there is quite as much of that vital substance in their 
interstitial cavities as we find in the greater portion of siliceo-reticular 
sponges, and frequently more in proportion than we find in many of 
them. It is also an error to conclude that there are no membranous 
tissues within them. The usual delicate interstitial membranes exist 
in their cavities to about the same extent as in other sponges. 
The author also says, in his character of Vitrea, “never con- 
taining formed horny matter, either fibrous, membranous, or gra- 
nular.”’ Now Hyalonema contains an abundance of horny matter, 
cementing the spicula together in the basal mass of the sponge, and 
also in the coriaceous envelope of the so-called “ glass rope”’ of the 
sponge, and Dactylocalyx has plenty of membranes in the interstices 
of the skeleton, and an extensive and elastic dermal membrane en- 
veloping the whole of the sponge. 
Prof. Thomson’s description of the characters of his proposed new 
order embraces very many more genera than he could possibly have 
contemplated when he wrote it. He says, ‘‘ The skeleton consists 
entirely of siliceous spicules, either separate (in fascicles or scattered) 
or anastomosing and combined into a siliceous network.” Let us 
now see what the effect of this very sweeping character will be :— 
Ist, under the head of spicules ‘‘ separate,” it will include the genera 
Hymeniacidon and Hymeraphia; 2nd, “ spicula in fascicles,” it 
will embrace ten other genera, viz. Geodia, Pachymatisma, Ecio- 
nemia, Dictyocylindrus, Polymastia, Ciocalypta, Tethea, Phakellia, 
Microciona, and Hymedesmia ; 3rd, “ combined into a siliceous net< 
