EEPORT ON THE TETRACTINELLIDA, 
399 
It will be particularly observed that although the distribution of the Hexactinellida is 
characteristically deep water, and of the Monaxonida just as characteristically shallow 
water, yet that the Monaxonida are really commoner in deep water of from 201 to 1000 
fathoms (in which depths the Hexactinellida reach their maximum) than the Hexacti- 
nellida themselves, and it is not till the depth of 1000 fathoms is passed that the 
Hexactinellids begin to prevail over the Monaxonids, finally, however, becoming much 
more numerous, in a little over the ratio of 2 : 1. It may here be added that the 
following proportion approximately represents the distribution of the Sponges at 
abyssal depths (over 1000 fathoms): — Number of Spongise : number of Hexactinel- 
lida : number of Monaxonida : number of Tetractinellida = 11 : 7 : 3 : 1. 
It is obvious that if the bathymetrical distribution of groups of organisms remained 
constant throughout all time (and it is too frequently assumed that this is the case), 
the value of such a table of curves as that given on p. 397 as a means for determining the 
depth at which strata containing sponge remains had been deposited would be very great. 
Zittel evidently assumes that the depth at which ancient sediments were deposited may 
be inferred from the nature of the associated fossil Sponges, and even Schulze, specu- 
lating on the sudden appearance of the Lithistida and Hexactinellida, argues that as : — 
“ Both groups are especially inhabitants of the deep sea [soj only in the former deep- 
sea deposits can one hope to find their fossil remains in any abundance.” When 
Vosmaer, however, supposes that the ancestral forms of Sponges originated in deep water 
he evidently assumes a change in bathymetrical distribution but in the contrary direction 
to that in which it has taken place. 
The question is evidently important enough to demand a few words of inquiry, and 
as the Hexactinellida are pre-eminently a deep-water group at present, they will best 
serve for investigation. The oldest known Hexactinellid is the Lyssacine Protospongia, 
which occurs in Cambrian slates, and though we have no evidence as to the depth at 
which the mud, out of which the slate originated, was deposited, one would probably not 
regard it as very great ; but leaving an instance so doubtful, we pass on to the Silurian, 
and there encounter a remarkable group of Hexactinellids, known as the Dictyospongidse ; 
the individuals of this group are not, for fossil Hexactinellids, rare ; many of them are of 
large size, and evidently lived under favourable conditions; the group ranges from the 
Silurian into the Carboniferous system, and is usually associated with sandstones or sandy 
shales. The beautiful examples from the Chemung sandstone of Devonian age occurring 
in North America attain a height of nearly a foot and a diameter of 5 or 6 inches. Here 
then we have an instance of an abundant group of Hexactinellids (Lyssacina according 
to James Hall) associated with rocks which were undeniably deposited in shallow water 
at no great distance from land, and such a plain instance to my mind altogether upsets the 
h3q)othesis that the existing bathymetrical distribution of this group is the same or even 
in a general way similar to that which it possessed in the past. 
